Abstract
Cystathionine synthetase was approximately 240 folds purified from rat liver by the following procedures: 1) acid treatment, 2) ammonium sulfate fractionation, 3) 1st DEAE-cellulose column chromatography, 4) treatment with phosphorylated cellulose, 5) Sephadex G-100 column chromatography and 6) 2nd DEAE-cellulose column chromatography. The purified preparation had no serine dehydratase activity. It was previously reported by us that crystalline serine dehydratase was of no cystathionine synthetase activity and anti-serine dehydratase serum did not inhibit cystathionine synthetase activity. In line with these facts, the data presented here indicate clearly that cystathionine synthetase is entirely different from serine dehydrase. The enzyme was inhibited by hydroxylamine and the inhibition was reversed by the addition of pyridoxal phosphate. The Km values for L-homocysteine and L-serine were 1.1×10-2M and 1.6×10-3M, respectively. The optimum pH was 8.6 in Tris-HCl buffer.
On the basis of the above findings, we improved the cystathionine synthetase assay of Mudd et al. By means of this technique, it was demonstrated that cystathionine synthetase of liver was repressed considerably in the rats maintained on a synthetic diet containing 0.16% of Lmethionine plus 0.274% L-cystine (sparing diet) in comparison with the animals on the diet containing 0.5% L-methionine (control diet).
This feedback inhibition of cystathionine synthetase by the end product, cystine, accounts well for the methionine sparing action of L-cystine for growth of rats established by Womack and Rose in 1939.
