Hydroxyphenylpyruvate Reductase From Cell Suspension Cultures Of Coleus Blumei Benth.

Cell suspension cultures of Coleus blumei Benth. producing high amounts of rosmarinic acid were used to study the biosynthetic pathway of this caffeic acid ester. One of the involved enzymes, the hydroxyphenylpyruvate reductase (HPPR), is characterized in this paper. HPPR catalyzes the NAD (P)H dependent reduction of p-hydroxyphenylpyruvate to p-hydroxyphenyllactate. The enzyme developed maximal activity at an incubation temperature of 37 °C and at a pH of 6.5 to 7.0. The reaction proceeded linearly for an incubation time of 60 min and up to a protein concentration of 0.2 mg per assay. As electron donor HPPR accepted NADH and NADPH with Km-values of 190 µm and 95 µm respectively. The enzyme reduced differently substituted hydroxyphenylpyruvates but not β-phenylpyruvate. The apparent Km-values for the various substrates were at 10 µM for p-hydroxyphenylpyruvate, at 130 µm for 3,4-dihydroxyphenylpyruvate and at 250 µM for 3-methoxy-4-hydroxyphenylpyruvate. HPPR was competitively inhibited by rosmarinic acid and pyruvate with Ki-values of 210 µM and 200 µM respectively. Caffeic acid, p-coumaric acid and cinnamic acid did not affect the enzyme activity but p-coumaroyl-CoA inhibited HPPR