Measurement of Serine Acetyltransferase Activity in Crude Plant Extracts by a Coupled Assay System Using

Serine acetyltransferase (SATase) (EC 2.3.1.30) catalyzes the formation of O-acetyl-L-serine (OAS) from L-serine in the presence of acetyl-CoA. A novel assay method was developed for measuring this enzyme activity in extracts from plant tissues. The assay consists of a coupled system in which the OAS formed is converted to cysteine by the addition of cysteine synthase (CSase) (EC 4.2.99.8). Cysteine thus formed is determined colorimetrically and serves as a measure for SATase activity. This method is rapid, simple and sensitive, and can be readily adapted for measurement of SATase activity in crude tissue extracts or homogenates. Key words: Cysteine biosynthesis — New assay system — Serine acetyltransferase — Sulfate assimilation. In higher plants, L-cysteine is a major portal for the bioproduction of organic reduced-sulfur compounds; therefore cysteine biosynthesis is one of the essential reactions in the metabolic pathway of plants. This anabolic reaction proceeds in two steps as follows: L-Serine + Acetyl-CoA —* • O-Acetyl-L-Serine (OAS)+CoA OAS + Sulfide —*- L-Cysteine+Acetat