‘Soluble’ Cysteine Desulphurase of Rat Liver as an Adaptive Enzyme

Abstract

IN a previous communication from this laboratory, results were presented which show that the enzymatic formation of hydrogen sulphide from L-cysteine in the liver of the rat occurs through two pathways¹. During the course of purification of the pyridoxal phosphate-dependent enzyme present in the non-particulate fraction of the liver (which we referred to as ‘soluble’ desulphurase), it has been observed that the purified enzyme which catalyses the formation of hydrogen sulphide and ammonia from L-cysteine is also responsible for the formation of ammonia from some other substrates and particularly from DL( + )-allo-cystathionine; the ratio of specific activities of the enzymatic preparations towards cystathionine and towards cysteine remained the same even after an approximately 300-fold purification. We concluded² that the soluble cysteine desulphurase was probably identical with the cystathionase-homoserine deaminase crystallized by Matsuo and Greenberg³; but, from the results of some inhibition experiments, we suggested that the mechanism of action of cystathionase appeared to consist of the deamination of substrates leading to the labilization of γ-substituents rather than the reverse mechanism proposed by Matsuo and Greenberg. Recently, Goswami et al.⁴ observed that the L-cysteine desulphydrase activity in an homogenate of chick liver (where the two pathways demonstrated in rat liver are also present⁵) was increased if L-cysteine or L-histidine were injected to the animals for 4 days, while other amino-acids (DL-methionine, L-arginine, L-glutamic acid) were not effective.