Cystathionine Synthase from Rat Liver: Partial Purification and Properties

Cystathionine synthase which has been purified about 1000-fold from rat liver has absorbance maxima at 280, 360, and 428 mμ. Treating the enzyme with cysteine apparently affects the removal of pyridoxal phosphate and destroys the enzyme activity. So does reduction with borohydride. However, in neither case is the spectrum affected. These observations suggest that pyridoxal phosphate may be bound to cystathionine synthase in an atypical fashion.Mercuric ions strongly inhibit the enzyme, but not in the presence of serine; hydroxylamine inhibits, but not in the presence of substrates. Other carbonyl reagents inhibit little if at all. Sulfate ions activate the enzyme.A new assay procedure for cystathionine synthase has been devised. In the presence of 5,5′-dithiobis-(2-nitrobenzoic acid), the enzyme catalyzes the degradation of cystathionine to serine and homocysteine. The rate of increase in absorbance at 412 mμ is a measure of enzyme concentration.