Characteristics of a 4-Hydroxycinnamate Hydroxylase Purified from Sorghum Leaves

A membrane-associated 4-hydroxycinnamate hydroxylase (p-coumarate hydroxylase) from green leaves of Sorghum bicolor has been purified by mercaptoethanol treatment, ammonium sulfate fractionation, and chromatography on hydroxyapatite and agarose 1.5m. Ascorbate (or reduced pyridine nucleotide) is an obligatory electron donor for the hydroxylation of 4-hydroxycinnamate, but not for p-cresol. The most highly purified fraction has a 260/280 ratio of approximately 1 and contains carbohydrate or other orcinol-reacting materials. The hydroxylase enzyme exists in series of aggregated forms at pH 6 ranging from about 60,000 to 1.5 million depending on the ionic strength, but even at high ionic strengths the bulk of the enzyme exists in relatively high molecular weight aggregates.