Pertanika 9(2), 219-224 (1986)

Polyphenoloxidase from Starfruit (Averrhoa carambola, L.)'

TENGKU ADNIN BIN TENGKU ADNAN, M. A. AUGUSTIN and HASANAH MOHD. GHAZALI

Department of Food Science,Faculty of Food Science and Technology,

Universiti Pertanian Malaysia,43400 Serdang, Selangor, Malaysia.

Key words: Polyphenoloxidase; starfruit; catechol.

ABSTRAK

Polifenoloksidase (PPO) telah diasingkan daripada belimbing besi (Averrhoa carambola, L.)dan ditulenkan melalui pemendakan aseton dan dialisis. pH optimum untuk pengoksidaan katekolyang dimangkinkan oleh PPO yang ditulenkan dengan aseton adalah 7.2. Kajian penyahaktifan habamenunjukkan bahawa enzim PPO adalah labil terhadap haba. Hayat-separa untuk penyahaktifanPPO pada 70C dan 75C masing-masing ialah 4.6 min dan 3.1 min. Didapatijuga bahawa malarMichaelis (K J dan V untuk katekol dengan PPO masing-masing ialah 0.061 M dan 20 U ml

ABSTRACT

Polyphenoloxidase (PPO) was isolated from starfruit (Averrhoa carambola, L.) and purifiedby acetone precipitation and dialysis. The optimum pH for the acetone purified PPO catalysedoxidation of catechol was found to be pH 7.2 Heat inactivation studies showed that the enzyme washeat-labile. Half-lives for PPO inactivation at 70C and 75C were found to be 4.6 min and 3.1 minrespectively. The Michaelis constant (K J and V or catechol with PPO was 0.061 M and 20 Uml1

respectively.

INTRODUCTION

Starfruit (Averrhoa carambola, L.) is con-sidered to be under the oxalidaceae or oxalisfamily. Like most other fruits and vegetables,the starfruit discolours during storage or when itis cut and processed. The discolouration leadingto browning is a consequence of non-enzymic aswell as enzymic reactions involving polyphenoliccompounds. The specific enzymes which takepart in browning reactions have been referred togenerally as polyphenoloxidases (Mayer andHard, 1979; Vamos-Vigyazo, 1981). (Enzymenumber 1.14.18.1 with the systematic name ofmonophenol, dihydroxyphenylalanine: oxygenoxidoreductase; Anon, 1973).

A considerable amount of work has beendone on the isolation and characterization of

polyphenoloxidase. It has generally been foundthat PPO is a relatively difficult enzyme toextract and purify (Mayer and Harel, 1979).Although there appears to be a wealth of infor-mation on the subject, the overall picture ofpolyphenoloxidase is not complete.

This is because (i) PPO acts on a greatnumber of substrates and catalyses more thanone reaction (ii) PPO can occur in multipleforms in individual species (iii) the physiologicalrole of the enzyme is not understood and (iv) ithas a complex role in food processing (Vamos-Vigyazo, 1981).

It is also clear that the characteristics ofPPO vary with the type of fruit. There are dif-ferences between PPO from different cultivars

Presented in part as a Poster at the 1 lth Malaysian Biochemical Society Conference, 9th - 10th September, 1985.

TENGKU ADNIN BIN TENGKU ADNAN, M.A. AUGUSTIN AND HASANAH MOHD. GHAZALI

and also differences in PPO isolated at differentstages of maturity of fruits and vegetables. Thishas become evident from reports on PPO frompeaches (Wong et al, 1971; Luh and Phit-hakpol, 1972; Flurkey and Jen, 1978), banana(Galeazzi and Sgarbieri, 1981; Galeazzi et al,1981; Jayaraman et al, 1982), cherries(Benjamin and Montgomery, 1973), guava(Augustin et al, 1985), pears (Halim andMontgomery, 1978; Montgomery and Petio-pakis, 1980), avocado (Kahn, 1975; Kahn, 1977)and mango (Park et al, 1980). The charac-teristics of PPO from a large variety of fruits andvegetables has been reviewed by Vamos-Vigyazo(1981).

There has been no work reported on star-fruit PPO. An understanding of the enzyme willhelp efforts in controlling the undesirablebrowning of the fruit and processed starfruitproducts. The present work involved the extrac-tion and purification of PPO from starfruit anda study of its enzymic activity using catechol as asubstrate.

MATERIALS AND METHODS

Materials

Starfruits were obtained from the areaaround Universiti Pertanian Malaysia. The ageof the fruit was estimated at 1.5 months. Thefruits had a yellowish-green colour.

Extraction and Purification of PPO

The method used was based on that ofGaleazzi et al, (1981). 100 g of longitudinallysliced fruit was homogenized in a blender for 30sec with 100 ml of 0.2M phosphate buffer (pH6.8) containing 1.5% polyvinylpyrrolidone(PVP) and 0.5% Triton X-100. Homogenateswere centrifuged at 12 000 rpm (25,500 g) for 15min at 4C (Du Pont RC-5B Centrifuge). Theenzyme remained in the supernatant (crudeextract).

Previous reports have shown that one of themajor difficulties encountered during prepara-tion of a soluble PPO is that of preventing

enzymatic phenol oxidation and polymerizationduring extraction. This may cause irreversibleinactivation of the enzyme and a resultant lowactivity of the extracted enzyme (Vamos-Vigyazo, 1981). In this work, PVP which is aphenol scavenger was added to the extractionbuffer during the isolation step. This minimizesenzyme denaturation by phenol oxidationproducts (Stelzig et al., 1972). Triton-X wasincorporated into the extraction mediumbecause it functions as a solubilization agent.

Purification of the crude extract involvedacetone precipitation and dialysis. All purifica-tion steps were carried out at 4C. The acetoneprecipitate was obtained by addition of 2volumes of cold acetone and recentrifugation at15 min at 12 000 rpm (25 500 g) at 4C. Theprecipitate was reextracted with one volume of0.2M phosphate buffer (pH 6.8). For the dialysisstep, the reextracted solution containing theacetone-precipitated enzyme was put into adialysis bag (Sigma Dialysis 'Sacks' 250-7U) anddialysed against 0.2M phosphate buffer (pH6.8). The dialysis was carried out at4C for48hrswith four changes of dialysing media.

PPO Assay

Enzyme activity was determined by measur-ing the increase in absorbance at 410 nm. Thereaction mixture contained 1 ml 0.05Mcatechol, varying amounts of enzyme extract and0.2M phosphate buffer (pH 6.8) in a finalvolume of 5 ml. The temperature of the assaywas 30C. The rate of the reaction was calculat-ed from the initial linear slopes of activity curves.One unit (U) of PPO activity was defined as theamount of the enzyme that increased the absor-bance by 0.001 min ~* under the conditions ofthe assay (Benjamin and Montgomery, 1973;Galeazzi et al, 1981).

The pH profile of the enzyme was deter-mined in th range of pH 5.0 pH 8.0 using0.2M phosphate buffer and the conditions ofthe assay described above. Specific activity(U mg ') was plotted against pH to get the pHoptimum.

220 PERTANIKA VOL. 9 NO. 2, 1986

POLYPHENOLOXIDASE FROM STARFRUIT (AVERRHOA CARAMBOLA, L.)

The thermal denaturation of the enzyme(acetone-precipitated extract) was obtained byadding 3 ml of the enzyme extract in test-tubesand heating at 70C, 75C and 80C respec-tively. After various heating intervals, 0.5 mlsamples were withdrawn and immediately cooledin ice-water (Park et aL, 1980; Wissemann andLee, 1981). It was then assayed for PPO activitywith 0.05M catechol as the substrate at pH 6.8under the conditions of the assay describedabove.

Determination of K jind V

The Michaelis constant (K ) and themaximum velocity (Vmax) were determined byvarying the catechol concentrations of the re-action mixture over the range 0.02M-0.10Mcatechol. The temperature and pH of the re-action mixture was 30C and pH 6.8 respec-tively. The data was plotted as 1/ activity against1/ catechol concentration.

Protein Determination

Protein concentration of the extracts wasdetermined using the Lowry method as given byLayne (1957). Bovine serum albumin was used asthe standard.

RESULTS AND DISCUSSION

Isolation and Purification of PPO

The results of the extraction and purifica-tion of starfruit PPO are given in Table 1. Thecrude extract had low specific activity. Purifica-tion by acetone precipitation and dialysis result-

ed in a 9.5 and 22 fold increase in the specificactivity.

Acetone precipitation has often been usedin extraction and purification steps for PPO.Flurkey and Jen (1978) found that activities ofpeach PPO were greatly enhanced by acetonepowder preparation. It was suggested that the20-fold increase in PPO activities in extracts ofacetone powder over that of fresh extracts ofpeach PPO was due to the presence of inhibitorysubstances in fresh extracts or possibly because ofaggregation of PPO isoenzyme forms duringacetone powder preparations (Flurkey and Jen,1978). The possibility of these factors cominginto play in the starfruit PPO preparation can-not be excluded. The further increase in specificactivity after dialysis also suggests the removal ofother inhibitory low-molecular weight sub-stances from the enzyme extract.

pH Profile

Initial experiments on the activity of thecrude extract showed that the pH optimum inthe range pH 5.0-8.0 was pH 7.3. The pHprofile of the acetone purified extract is given inFigure 1. The pH optimum of this extract wasfound to be pH 7.2. It has been stated that thepurity of the enzyme extract can affect the pHoptimum (Stelzig et aL, 1972) but this was notevident in the case of the crude extract and aacetone purified extract from starfruit. The pHoptimum of most PPO's studied have beenshown to be in the range pH 5.0 7.0. It has alsobeen found that pH optimum of the enzyme canbe influenced by the type of phenolic substratebeing oxidized. Differences in pH optima can

TABLE 1Extraction and purification of starfruit PPO

Stage Proteinmgrnl"1

Specific activityU mg"1

Purificationfold

Crude extract

Acetone precipitate

Dialysed enzyme

14.2

0.65

0.19

2.6

24.7

56.7

1.0

3.1

1.1

1

9.5

22

PERTANIKA VOL. 9 NO. 2, 1986 221

TENGKU ADNIN BIN TENGKU ADNAN, M.A. AUGUSTIN AND HASANAH MOHD. GHAZALI

25

20

I 15

i 10

/ \

70 7.2

pH

Fig. 1: pH profile of the acetone purified extractfrom Starfruit.

also occur when PPO is obtained from differentcultivars of the fruit (Mayer and Harel, 1979).

A pH optimum of 7 has been found for anumber of other PPO preparations. Amongsome of them with a pH optimum of 7 for PPOactivity are PPO from certain varieties of apple,pear, sweet cherry, mandarin, banana, mango,mushroom and green chilly (Vamos-Vigyazo,1981). However, it should be borne in mind thatpH optimum as well as the relationship betweenactivity and pH can differ according to genera,cultivar and substrates (Vamos-Vigyazo, 1981).Changes in pH profile of PPO activity can some-times be accompanied by activation of theenzyme in the neutral to alkaline region of thepH curve. This is due to partial denaturation of

the enzyme and/or conformational changeswhich result in shifts in its pH optimum (Mayerand Harel, 1979).

Thermal Inactivation of Starfruit PPO

Figure 2 shows the results of the heat inacti-vation of the acetone purified PPO at 70C and75C. As expected, the rate of enzyme inactiva-tion increased with increasing temperature.Heat inactivation at 80 C for 2 min resulted in a95% loss of enzyme activity.

100

7 50r

I

108 10

TIME (MIN)

Fig. 2; Thermal Inactivation of Starfruit PPO( 70C; 75C).

The linearity of the plots of log activityagainst time showed that heat inactivationfollowed first order kinetics. The half lives forheat denaturation at 70C and 75C were foundto be 4.6 min and 3.1 min respectively. Studiesinto the heat inactivation of PPO from mangopeel and mango (Park et aLf 1980; Katwa et ai,1983), grape (Wissemann and Lee, 1981, Lee etal., 1983), pear (Halim and Montgomery, 1978)and cherries (Benjamin and Montgomery, 1973)have shown first order kinetics of denaturation.

A comparison of the heat lability of starfruitPPO with PPO from other sources show thatstarfruit PPO is more susceptible to heatdenaturation than PPO extracted from De

222 PERTANIKA VOL. 9 NO. 2, 1986

POLYPHENOLOXIDASE FROM STARFRUIT (AVERRHOA CARAMBOLA, L.)

Chaunac grapes, d'Anjou pears, mango andmango peel. Half lives for heat inactivation ofDe Chaunac grapes were about 3.5 min and 5.4min at 75C and 70C respectively (Lee et al,1983). PPO from d'Anjou pears were even moreresistant to heat denaturation as shown by halflives of 6.3 min and 11.7 min at 75C and 70Crespectively (Halim and Montgomery, 1978).Katwa et al. (1983) found that PPO from mangopeel lost 50% of its activity by exposure to 75Cfor 16 min.

~

20

3.20

b.10