The Oxidizing Enzymes of Sugarcane: Tyrosinase (Polyphenol oxidase)

Abstract

A study was made of the distribution and properties of tyrosinase (polyphenol oxidase) in sugarcane. The enzyme was extracted with phosphate buffer (pH 7) from freeze-dried tissues which had been ground to pass a 60-mesh screen. Tyrosinase was assayed spoctrophotometrically at 390 mµ by measuring the optical density increase of a buffered mixture of catechol and enzyme. Fractionation of cane extracts with ammonium sulfate showed that tyrosinase is precipitated readily from 20- to 70-percent saturation. The richest source of the enzyme was meristematic tissue. Considerable activity was also obtained from leaves — 1 and 0, whereas only traces of the enzyme were present in both 8 to 10 and 1 to 3 nodes and internodes. Tyrosinase preparations were heat-sensitive, losing most of their activity within 2-1: hours of extraction at room temperature (28-29°C.) and laboratory temperature (19-21°C). In contrast, to cane peroxidase, no recuperative capacity was evident after inactivation by boiling. Substrates acted upon by tyrosinase included tyrosine, catechol, DOPA (3,4, dihydroxyphenylalanine), pyrogallol, guaicol, resorcinol, hydroquinone, and paracresol. No reaction was observed with phenol or metacresol. Optimum temperature was about 21°C, and optimum pH was 7.5. Apparent Km was 4.0 X 10-3 mols of catechol per liter. Thiourea and hydroxylamine markedly inhibited the enzyme at concentrations of 1 umol/ml.  Cysteine, ascorbic acid, and cyanide caused virtually complete inhibition at this concentration. Carbon monoxide likewise inhibited. Tyrosinase which was inactivated by KCN was reactivated following prolonged dialysis against distilled water and addition of copper. No other metal tested (molybdenum, manganese, zinc, iron, magnesium) served to reactivate the catalyst. Possible roles and significance of cane tyrosinase are discussed.