Polyphenol oxidase extracted from oil bean (Pentaclethra macrophylla) seeds was purified 165-fold over the crude enzyme extract. The apparent molecular weight of the enzyme by gel filtration was 110.8 kf9.0 k while SDS-PAGE indicated a single species of molecular weight 28.0 k. A copper content of 1.9 mg g-' corresponds to one copper atom for each of the four subunits. The purified enzyme oxidised pyrogallol, catechol, 4-methylcatechol and Ldihydroxyphenylalanine but had low activity towards tyrosine. p-Cresol, caffeic acid and cholorogenic acid were not oxidised. Thio-compounds were strong inhibitors of the enzyme. The phenolic compounds tyrosine, resorcinol and orcinol inhibited catechol oxidation but became oxidised in the process.Key words : Pentaclethra macrophylla, oil bean seeds, polyphenol oxidase, purification, properties.Polyphenol oxidase (PPO) (o-diphenol : 0, oxidoreductase, EC 1.10.3.2) is a copper-containing enzyme which catalyses the o-hydroxylation of monophenols to o-diphenols and the oxidation of o-diphenols to 0-quinones. The enzyme can be found in a variety of species and, though it contributes to the discolouration of plant materials on injury, its functions in higher plants remain unclear (Vaughn and Duke 1984). Work on higher plant PPO is handicapped by the apparent difficulties in isolation and purification of the enzyme due to protein-quinone interactions and binding of enzyme to particles during grinding and fractionation (Mayer and Hare1 1979). Recent attempts to use affinity and hydrophobic chromatography on mushroom tyrosinase led to the discolouration of the matrix due to oxidation of coupled ligands (Ingebrigsten and Flurkey 1988). Apart from problems of isolation and purification, there is controversy over the number and type of polypeptide chains in higher plant PPO (Mayer 1987).The seeds of P macrophylla are eaten as a form of salad. The colour ranges from green to brown due to * Present address Department of Biochemistry, University of Port-Harcourt, Port-Harcourt, Nigeria. browning reaction. As these reactions cause loss of nutritive value of food (Jood et af 1987), it was decided to purify and characterise the enzyme from these seeds in order to control the browning. Table 1 shows the purification profile of the enzyme. Attempts to isolate the enzyme by ion-exchange chromatography from dialysed ammonium sulphate precipitates using a long column of DEAE-cellulose (30.0 cm x 2.0 cm id) and gradient elution were unsuccessful. However, stepwise elution of the buffer enzyme extract on a squat column (9.5 cm x 5.0 cm id) gave the enzyme in high purity.The enzyme was purified 165-fold with 20 % recovery of initial enzyme activity. The purified enzyme and bovine serum albumin (BSA) were separately electrophoresed in 75 g kg-' polyacrylamide gels in tris-glycine buffer, pH 8.3, and located by staining. The purified enzyme revealed a single band of mobility of 0.92 relative to BSA.Gel filtration of the purified enzyme according to Andrews (1 964) was with Sephadex G-200-120 c...