RESPIRATION IN THE ORANGE

A STUDY OF SYSTEMS RESPONSIBLE FOR OXYGEN UPTAKE

BY A. AZIZ HUSSEIN

(From the Diwision of Fruit Products, College of Agriculture, University of California, Berkeley)

(Received for publication, March 23, 1944)

In a study of respiration in the orange (Citrus sinensis) an attempt has been made to determine the type of oxygen-absorbing system present in the various tissues of the orange fruit, with particular emphasis on the flavedo. Oranges were classified as a peroxidase fruit by Onslow (1). Recently Davis (2) prepared peroxidase extracts from the different tissues of the navel orange and reported highest values in the flavedo. Qualitative tests with guaiacol and benzidine have confirmed this observation. In the present study, measurements of oxygen uptake by the Warburg technique show that a system resembling cytochrome oxidase is involved. Dehydro- genase-like activity, as measured by methylene blue reduction, has also been studied.

Meihods

Respiration measurements were made with the customary Warburg manometer and conical reaction flasks. The Qor or c.mm. of 02 consumed was calculated on the basis of 1 mg. of dry weight per hour. The oranges were supplied by the Citrus Experiment Station at Riverside. Weekly samples were received from the same tree and stored at 0”. The fruit was washed with cold water, adhering water removed, and the tissue desired removed with a sharp blade. Flavedo samples were taken from the middle, to avoid tissue with a greenish tint. Cross-sections with a thickness of approximately 0.7 mm. were made by free-hand slicing. The sections were rinsed with distilled water’ before transference to the Warburg vessels which contained 2 ml. of 0.2 M phosphate buffer. The samples weighed 200 mg. when fresh, containing about 50 mg. of dry weight.

Dehydrogenase activity was studied by methylene blue reduction in Thunberg tubes. The juice was prepared by mincing the sample, freezing and thawing, grinding with sand in a mortar, and finally pressing in a hand press. The juice was mixed then with infusorial earth and filtered through asbestos. The clear juice was decolorized with norit A, and again filtered. The clarified juice was kept under toluene at 0”. The final volume in the tubes was 2 ml., consisting of 0.2 ml. of 0.02 per cent methylene blue,

* The distilled water in these solutions was redistilled once in Pyrex. 201

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202 RESPIRATION IN ORANGE

0.5 ml. of the clarified tissue juice, and 1.3 ml. of 0.2 M potassium phosphate. The tubes were evacuated for 1 minute with a Nelson pump and allowed to stand for 20 minutes to reach room temperature before mixing. A tube containing all the reagents after decolorieing was used for comparison. As the reaction was found to be catalyzed by light, the reduct.ion period was measured under artificial light with a 75 watt Mazda lamp.

EXPERIMENTAL

Determindion of Respiratory Quotient-The respiratory quotient WM determined in 0.2 M phosphate buffer of pH 5.0. In six determinations, the average was 1.33 at the start and 1.09 after 2 hours.

Foci. 1 FIG. 2

FIG. 1. Comparative activity of different tissues. Curve A, flavedo tissue; Curve B, albedo tissue; Curve C, juice sacs; Curve D, carpellary membranes.

FIG. 2. Effect of size of flavedo section on oxygen uptake. Curve A, 3 mm.; Curve B, 6 mm.; Curve C, 12 mm. in diameter.

General Characteristics of System. The Sample-Flavedo sections prepared in the same manner, but obtained from different locations on the orange, showed different oxygen uptake, with highest activity near the stem end, and lowest near the blossom end. Qo, values were 1.35, 1.17, and 0.92 respectively. Variability among oranges is of considerable magnitude, ranging from 1.60 to 2.10 for twelve navels and 1.10 to 1.55 for twelve Valencias taken at random. Maturity, elapsed time, and the uncontrollable variables are doubtless responsible for this. Comparative activities for the different tissues are shown in Fig. 1 for navels. In each, the flavedo has the highest and the carpellary membranes the lowest activity. An increase was noted in oxygen uptake with decreasing size,

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A. A. HUSSEIN 203

but similar trends were obtained for the values taken at comparable time intervals (Fig. 2). Although decreasing the length of the cross-sections resulted in an increase in &on, the activity was lost if the tissue was finely ground with sand.

WashinS-When flavedo sections are washed in running water, their activity decreases gradually and finally ceases completely after 24 hours. When the tissues were washed for 24 hours with mechanical stirring and with a stream of air passing through the liquid, between 30 and 40 per cent of the initial activity was retained.

To determine whether this difference in activity is associated with fermentation, flavedo sections were left in the Warburg vessels for 22 hours under an atmosphere of purified nitrogen. An average QcoI of 1.66 for the first 3 hours was observed. The original Qol was 1.33. After fermentation

I I I I I I L 4

t.“H ’ I0

FIG. 3. Effect of pII on the oxygen uptake of flavedo. Curve A, 1st hour; Curve B 2nd hour.

was allowed to continue for 22 hours, the value for QO, dropped to 0.16 c.mm. for the 1st hour and no further oxygen uptake was observed after 2 more hours.

Heat-When flavedo sections were heated for 2 minutes in boiling water or buffer, neither the tissue alone nor the fluid showed any oxygen uptake. Heating was also conducted in an atmosphere of purified nitrogen, but the power of oxygen uptake was lost.

Effect of pH-The pH of the expressed juice of both flavedo and albedo tissues ranges from 4.9 to 5.4. When the oxygen uptake of flavedo sections was determined at different pH values between pH 3.0 and 7.0, an optimum near pH 5.0 was observed for both varieties. Above pH 7, the oxygen uptake remains relatively constant, but rises slightly near pH 8 (Fig. 3).

E$ect of Substrates. Catechol-To establish the presence or absence of

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204 RESPIRATION IN ORANGE

polyphenolase in the flavedo, the effect of catechol was determined at the natural pH, 5.1, and at pH 8.15. The side arm of the flasks contained 0.2 ml. of 1 per cent catechol in’ aqueous solution. The initial oxygen uptake of the tissue was determined for 30 minutes before the catechol was tipped into the flasks. This caused no increase in the oxygen uptake at either pH.

Ascorbic Acid-The oxygen uptake of flavedo tissue in the presence of 2 mg. of ascorbic acid per ml. was compared with aqueous solutions of ascorbic acid at the same concentration, at different pH values (Table I). To ascertain whether ascorbic acid is partially oxidized, an extract was prepared by the method of Krishnamurthy and Giri (3). 1 ml. of extract was added to 1 ml. of buffer of pH 6.75, containing 5 mg. of ascorbic acid. The pH was 6.3 after the material was mixed. A buffer at this pH with the same quantity of ascorbic acid was used for comparison. The control vessels consumed 192 c.mm. of oxygen in 45 minutes, while there was

TABLE I

Oxygen Uptake by Flavedo with Ascorbic Acid

PH

8.05 6.50 5.00

Cam. oxygen consumed per 100 mg. dry weight

Ascorbic acid control Flavedo control Flavedo ;iv ascorbic

242 116 177 177 140 159 126 149 142

none in the presence of the extract, indicating apparently complete protec- tion of the ascorbic acid.

p-Phenylenediamine-Experiments were conducted in phosphate buffer, pH 6.5; the side arm contained 0.2 ml. of a solution of the reagent (1.92 mg.) in 0.1 M buffer in 50 per cent alcohol. A blank contained the same reagents without the tissue. Autoxidation was negligible, about 5 cmm. per hour. At the end of the experiment, both tissue and fluid were blue, and a 3-fold increase in oxygen uptake was observed. After 1 hour, p-phenylenediamine was added to one flask which had received none at the beginning. An increase in the oxygen consumption was also observed (Fig. 4).

The effect of p-phenylenediamine on the Qo, of the heated tissue, which had been boiled for 2 minutes, and on ground flavedo tissue was then determined. Oxygen consumption by the boiled tissue was similar to that of the blank, while the ground tissue, which did not consume oxygen in other experiments, gave values comparable with those of intact tissue alone, and the fluid was blue. The results are shown in Table II.

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A. A. HUSSEIN 206

Experiments were also conducted to test the effect of the dye in the presence of potassium cyanide. In the presence of cyanide (0.01 M) neither tissue nor fluid became blue, and inhibition was about 75 per cent (Table III). The sensitivity to cyanide did not change when the test was repeated over an 8 week period.

Cytochrome c-The addition of 0.4 ml. of reduced cytochrome c solution, prepared from ox heart (4), to flavedo sections at pH 8,6.5, and 5.0 caused an increase in oxygen uptake in all cases (Fig. 5). Cytochrome c is not autoxidised at these pH values and no blank, the cytochrome alone, was needed.

TABLE II Oxygen Uptake by Flavedo urith p-Phenylenediamine

Tissue c.mm. oxygerln$;~~h~ pet loo log.

Experiment 1 Experiment 2

Fresh,intact...................................... 128 98 (I “ withdye............................ 378 248 6c ground..................................... 0 0 “ u with dye . . . . . . . . . . . . . . . , . . . . . . 123 97

Boiled, with dye.. . . . . . . . . . . . . . . . . . 0 O-5

TABLE III Effect of Cyanide on Oxygen Uptake with p-Phenylenediamine

Cmm. oxygen co~+omed per 100 mg. dry weight

Fhvedo Flavedo with dye Fhvedo with dye and cyanide

107 320 81 108 294 79

H@roqu&one--Hydroquinone was added at the beginning and also during the course of the experiment. The side arm contained 0.2 ml. of 0.1 M aqueous hydroquihone. In neither case was an increase in uptake or brown discoloration noted. In three determinations, with and without hydroquinone, the average values after 2 hours at pH 7.0 were 0.77 and 0.75 respectively, and 0.66 and 0.61, when added after 1 hour. The oxidation of hydroquinone is catalyzed by cytochrome oxidase, and is a function of both the oxidase and cytochrome c (5). Since the hydro- quinone was not oxidized, one may infer the absence of cytochrome c.

Other Possible SubstratesThe following substances were also tested: 0.1 M solutions of malic, citric, and succinic acids, orange oil, citral and

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206 RESPIRATION IN ORANGE

limonene, citriq, riboflavin (with and without tissue dialysis), and glucose, fructose, maltose, galactose, and sucrose. None of these showed any effect on oxygen uptake by the flavedo, with the possible exception of succinic acid, for which a slight increase, about 10 per cent, in the Qo, was observed.

E$ect of Inhibitors. Potassium Cyanide-Cyanide has an inhibitory effect ranging from 80 per cent at 10d2 M to 8 per cent at 10v3 M at pH 5.0, indicating a residual respiration of about 20 per cent (Fig. 6). It was also

30 TIPIE % MIN.

w 30 TIME P, “IN.

90 no

FIG. 4 FIG. 5 FIG. 6

FIG. 4. Effect of p-phenylenediamine on oxygen uptake in flavedo. Curve A, tissue alone; Curve B, with p-phenylenediamine added after 60 minutes; Curve C, with p-phenylenediamine added at the start.

FIG. 5. Effect of added cytochrome c on oxygen uptake. Curves A and At at pH 8.0, without and with cytochrome; similarly Curves B and BI at pH 6.5; also Curves C and Cl at pH 5.0.

FIG. 6. Effect of cyanide concentrat,ion on oxygen uptake of flavedo (pH 5.0).

found that t,he extent of inhibition depends on the pH, being about 80, 75, and 50 per cent at pH values of 5.0,6.5, and 8.0 respectively.

Hydrogen Sulfide-To determine the effect of hydrogen sulfide, the t.issues were allowed to respire for 1 hour in phosphate buffer at pH 5.1, containing 0.01 M sodium sulfide. About 0.2 ml. of the fluid was trans- ferred to the side arm containing the alkali mixture (KMn04 and KI), and then HgCl2 was mixed with the fluid. The flasks were shaken until constant readings of the manometer were obtained. This experiment was conducted only with Valencia oranges; the inhibition in two experi- ments was 75.5 and 80.3 per cent, respectively.

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A. A. HUSSEIN 207

Sodium Azide-With 10v2 M aside, an inhibition of about 80 per cent was observed at pH 5.0, and 8 per cent at pH 8.0.

Carbon Monoxide-This experiment was conducted in light and in the dark, on tissue from the same orange, with a mixture of 80 per cent carbon monoxide and 20 per cent oxygen. In each experiment, the average of data from two flasks was used (Table IV). The inhibition is clearly re- versible with light.

Spectroscopic Examination of Tissues-An attempt was made to observe the charact,eristic bands of cytochromes in the flavedo tissue. When a suspension of finely ground tissue in water was examined spectroscopically, no bands were observed. However, when sodium hydrosulfite was added to the suspension, a band at 563 rnp was easily recognized. When flavedo slices were examined, the band could be observed only after steeping the slices in hydrosulfite. Cytochrome b exhibits a band at 564 rnk, and the oxidase at 582 rnp (6). To determine whether the band was due to cyto-

TABLE IV

Effect of Carbon Monoxide on Oxygen Uptake by Flavedo

Cam. oxygen consumed per 100 mg. dry weight

In daylight In dark Affiait ratio

&

In air In gas mixture In air In gas mixture

140 143 157 121 14.1 135 131 122 95 14.4

*KG residual respiration co 1 -. residual respiration xo2*

chrome b, its position was checked against the known cytochrbme b corn- ponent in a yeast suspension. To the knowledge of the writer, none of the cytochromes has been observed before in a colored portion of the plant.

The characteristic bands of cytochrome c (550 and 520 mp) were not observed. On the other hand when reduced cytochrome c was added to the flavedo suspension in the presence of Na&204, t,he band at 550 rnp was plainly visible, indicating that the failure to observe the band before was not due to a masking effect.

When cytochrome c was added in the absence of Na2S204, the bands gradually disappeared, indicating a change from the reduced to the oxi- dized form, but when the tissue was heated before the cytochrome solution was added, the bands remained visible. This indicates that a heat-labile substance is responsible for the oxidation of added cytochrome c.

Success in viewing cytochrome bands depends upon t,wo factors. As

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208 RESPIRATION IN ORANGE

the concentration of cytochromes is extremely low, a thick suspension is essential. This requires an intense beam to permit adequate transmitted light for visual inspection in the spectrometer. As’light source, a Wotan lamp (6 volts, 5 amperes) was used, with a large condensing lens, and diaphragm, as used in a microscope assembly. Two instruments were tried, a small direct vision spectroscope, and a large Bausch and Lomb spectromet,er. For best results on the bands in situ, two or three slices or more of flavedo steeped in hydrosulfite should be clamped together.

Observations on Reduction of Methylene Blue-Frankenthal (7) studied the dehydrogenase activity in the clarified juice of Palestinian orange peel,

TABLE V Effect of pH on Methylene Blue Reduction Time

Orange

Valencia

Navel

-

-

PH

8.4 8.2 7.9 6.9 5.9 8.2 7.9 6.9

Albedo

min.

16.5 15.7 19.0 96

600 12.5 15.5 60

TABLE VI

-

_-

-

Flavedo Whole rind

min. min.

10.5 12.5 11.4 13.5 12.5 17.2 52 70

330 450 8.9 11.5

10.0 14.0 33.5 43

Effect of Heat on Methylene Blue Reduction Time for Valencias

PH

7.0 9.0

Heated juice Unheated juice

m&6. min.

205, 220 56, 54 34,. 35 12.6, 12

and attributed the observed activity to dehydrosscorbic acid and its stable oxidation product, diketogulonic acid. Flavedo juice of California oranges was clarified with several types of plant and animal charcoals and norit A gave a juice with highest activity. The methylene blue reduction was found to be catalyzed by light. Since Frankenthal did not define her light conditions, results cannot be directly compared. Direct sunlight caused instantaneous reduction at pH 9. Measurements were made with artificial light at room temperature. With increase in pH, the reduction time was decreased.

The activity was higher in flavedo than in albedo juice, and navel orange juices were more active than those of Valencias (Table V).

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A. A. HTJSSEIX 209

h’fect of Heat-Flavedo juice was heated to boiling in a water bath, held for 2 minmes, and quickly cooled. After cooling, the solutions were darker yellow and a slight turbidity developed. Times required to de- colorize methylene blue are given in Table VI. They are averages of two determinations in each case. At both pH values, the activity was decreased by about two-thirds. This is in agreement with Frankenthal (7). This implies that much of the observed reduction of methylene blue is not enzymatic.

E$ect of Cyanide--Frankenthal (7) also reported that the reduction of methylene blue was accelerated by potassium cyanide. To 13.5 ml. of navel flavedo juice, 1.5 ml. of 0.1 M cyanide were added, and the reduction time at pH 9.0 was reduced from 8.5 to 5 minutes, confirming the earlier results (7).

DISCUSSION

Results of the present study appear to show that ascorbase2 and poly- phenolase are absent or non-functioning in the tissues of the orange fruit. Enzymes comparable in many respects with peroxidase, indophenol oxidase (cytochrome oxidase), and dehydrogenase have been shown to be present, though some doubt exists in the case of dehydrogenase, as this may be supplemented by a relatively heat-stable system capable of reducing methylene blue. This has been attributed to ascorbic acid (7).

Measurements of oxygen uptake could not be obtained on tissue extracts or homogenized tissue. Intact tissue was needed. There is considerable inherent variation within oranges selected at random. Comparable varia- tion exists in such fruit with respect to soluble solid content (8). The observation that the highest activity occurs in the flavedo tissue is in agreement with distribution of peroxidase activity (2) and methylene blue reduction (7). Values for Qo, for flavedo under optimal conditions varied from 1.10 to 2;lO c.mm. per mg. of dry tissue per hour.

The destruction of the activity by heat and the occurrence of an opti- mum in the pH range 3 to 7 indicate that the observed activity is enzymatic in nature. The optimum pH for the enzyme is on the acid side of neu-

2 Further study of the situation with respect to ascorbase suggests that this con- clusion needs modification. The validity of the ascorbic acid control may be ques- tioned because of the absence of protective factors, of which citric acid may be one (3). The flavedo control, based on average analyses, may contain up to 0.4 mg. of ascorbic acid, and the extent to which this and additional substrate may be protected, or oxidized, depends on the validity of the controls. It appears that the rate of oxida- tion is independent of the concentration at this and higher levels, owing to saturation of the enzyme. Regardless of this uncertainty, the theoretical maximum for com- plete oxidation of the ascorbic acid is less than 30 c.mm., while the flavedo control, on the same basis, has an uptake of 140 cmm.

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210 RESPIR4TIOS IN ORANGE

trality, while autoxidation is greater on the alkaline side. In these studies cyanide inhibition at pH 5.0 is about 50 per cent at 10e3 M, and 80 per cent at 10m2 M. The respiration of Nitella (9) is about 37 per cent inhibited at 1O-3 M concentration, of barley leaves (10) 65 per cent, of carrots 61 to 72, and of tea leaves (11) 58 to 88. In such tissues as carrot and tea leaf, in which cytochrome. oxidase has been reported, inhibition reaches a maxi- mum at 10m2 M cyanide, and is by no means complete. This is in contrast. with cytochrome oxidsse in the absence of other enzymes, when 5 X 1O-4 M

cyanide produces complete inhibition. Since inhibition increases with increase in cyanide concentration (Fig. S), the writer favors the suggestion (11) concerning the competition for organic iron between the enzyme and cyanide.

The constant for the ratio of the affinity of the orange enzyme for oxygen and carbon monoxide, namely 14, is close to some of the recorded values, 9 to 1.2 for carrots (12) and 15 for tea leaf (11).

Cytochrome b was observed only in the presence of hydrosulfite, and neither cytochrome a nor c could be detected.

Frankenthal (7) attributed the dehydrogenase-like activity in the clarified peel juice to the ascorbic acid oxidation products, dehydroascorbic acid and diketogulonic acid, and the results of this investigation with respect to the effect of cyanide, pH, and heat are in general agreement. The ascorbic acid could furnish the basis for dehydrogenase-like activity required for the completion of a respiratory cycle involving cytochrome b and a cytochrome-like oxidase.

SUMMARY

Measurements of oxygen uptake have been made on orange tissues by the Warburg technique. The flavedo shows the highest activity with an optimum at about pH 5.0.

The effect of inhibitors and numerous possible added substrates was noted, from which it was concluded that a system involving cytochrome oxidase is responsible for a major part of the oxygen uptake.

The presence of cytochrome b was identified spectroscopically by means of its characteristic absorption band at 563 ml.r. This was observed only after reduction with hydrosulfite.

Dehydrogenase activity was studied by methylene blue reduction in Thunberg tubes. A somewhat high proportion of thii activity (about 30 per cent) appears to be due to a relatively heat-stable system.

The writer wishes to express his sincere appreciation to Dr. M. A. Joslyn with whom this study was initiated. He also feels greatly indebted to Dr. G. Mackmney for his particular interest and for guidance in the

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A. A. HUSSEIN 211

spectroscopic work. Thanks are also due to Dr. W. V. Cruess and Dr. L. A. Hohl for their profitable suggestions.

BIBLIOGRAPHY

1. Onslow, M. W., The principles of plant biochemistry, Cambridge, pt. 1,132 (1931). 2. Davis, W. B., Am. J. Bot., 29,252 (1942). 3. Krishnamurthy, P. V., and Giri, K. V., J. Zndian Chem. Sot., 18,7 (1941). 4. Keilin, D., and Hartree, E. F., Proc. Roy. Sot. London, Series B, 127, 167 (1939). 5. Stotz, E., Sidwell, A. E., Jr., and Hogness, T. R., J. Biol. Chem., 124,733 (1938). 6. Stotz, E., A symposium on respiratory enzymes, Madison (1942). 7. Frankenthal, E., Enzymologia, 6, 287 (1939). 8. Bartholomew, E. T., and Sinclair, W. B., California Citrograph, 24,362 (1939). 9. Ross, E., Am. J. Bat., 26,458 (1938).

10. James, W. O., and Hora, R. B., Ann. Bat., 4, 107 (1940). 11. Deb, S. B., and Roberts, E. A. H., Biochem. J., 84,1507 (1940). 12. Marsh, A., and Goddard, D. R., Am. J. Bot., 26,767 (1939).

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A. Aziz HusseinFOR OXYGEN UPTAKE

STUDY OF SYSTEMS RESPONSIBLE RESPIRATION IN THE ORANGE: A

1944, 155:201-211.J. Biol. Chem. 

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