BIOSYNTHESIS OF P-HYDROXYBENZOATE FROM P-COUMARATE AND P-COUMAROYL-COENZYME-A IN CELL-FREE-EXTRACTS OF LITHOSPERMUM-ERYTHRORHIZON CELL-CULTURES

The enzymatic formation of p-hydroxybenzoate from p-coumarate in cell-free extracts of cell cultures of Lithospermum erythrorhizon Sieb. et Zucc. was investigated. p-Coumaroyl-coenzyme A (p-coumaroyl-CoA) is the activated intermediate in this biosynthetic reaction. It is formed by an ATP-, Mg2+-, and CoA-dependent 4-hydroxycinnamate:CoA ligase reaction. p-Coumaroyl-CoA is oxidized and cleaved to p-hydroxybenzoyl-CoA and acetyl-CoA in a thioclastic reaction in which NAD is an essential cofactor. These CoA esters are rapidly hydrolyzed to acetate and p-hydroxybenzoate, probably by thioesterases. The enzymes involved in the formation of p-hydroxybenzoate are soluble. p-Hydroxybenzaldehyde is not an intermediate in this conversion, and S-adenosylmethionine and uridine-5'-diphosphoglucose do not enhance formation of p-hydroxybenzoate in our system.