Studies on the nature, inducibility, and assay of the threonine and serine dehydrase activities of rat liver.

Threonine dehydrase (deaminase), which catalyzes the conversion of I,-threonine to a-ketobutyrate and ammonia, has been observed in crude extracts prepared from a number of different organisms (1-13). Frequently these same extracts also act on L-serine to form pyruvate and ammonia. Table I summarizes the available data on the serine and threonine dehydrasel activities in crude extracts from various sources. There is good evidence that a single enzyme is responsible for threonine and serine dehydrase activity in Neurospora crassa (4)) the rumen microorganism LC (5), and in Escherichia coli (2) although the situation in Escherichia coli is complicated by the existence of at least two threonine dehydrases (3). In the sheep it seems clear that there are two distinct enzymes; there is a serine dehydrase which does not attack L-threonine (7), and a threonine dehydrate which also has some serine dehydrase activity (8). There are three points of particular interest with regard to the activities in rat liver. The variation in reported activities is extraordinarily large. We shall show that this is probably due to variation in the assay procedure. Sayre, Jensen, and Greenberg (9) reported that the threonine, but not the serine dehydrase activity, was substrate-inducible both in vivo, and in the isolated, perfused liver. We shall present evidence to suggest that this enzyme is not substrate-inducible. Selim and Greenberg (11) established that the serine dehydrase and cystathionine synthetase activities of rat liver were due to the same protein; our data suggest the possibility that the serine and threonine dehydrase activities, and therefore also the cystathionine synthetase activity, are associated with the same protein.