Isolation and Properties of a Homogeneous Preparation of Cystathionine Synthetase-l-serine and L-threonine Dehydratase.

Selim and Greenberg (1, 2) achieved a considerable degree of purification of L-serine dehydratase (L-serine hydro-lyase (deaminating), EC 4.2.1.13) from rat liver and demonstrated that this protein preparation contained the cystathionine-synthesizing activity of the liver (L-serine hydro-lyase (adding L-homocysteine), EC 4.2.1.21). These workers (2) also observed activity of their enzyme preparation on L-threonine. Subsequently, the work of Goldstein, Knox, and Behrman (3) indicated that the L-threonine dehydratase activity of rat liver was a function of the same enzyme protein. Pitot, Potter, and Morris (4) demonstrated a large increase in the threonine and serine dehydratase activities of the livers of rats fed a high protein diet. This observation was confirmed by Goldstein et al. (3). This ability to increase the content of the enzyme in the liver offers a decided advantage in its purification. From livers of rats fed a high protein diet, we have succeeded in obtaining enzyme preparations apparently consisting of a single homogeneous protein. Various properties of the purified enzyme have been studied.l An observation of considerable interest is that the /3-hydroxyl group is not an essential characteristic for substrate activity. Chloride, for example, can be substituted for the hydroxyl group.