Transsulfuration in higher plants Partial purification and properties of β-cystathionase of spinach

Abstract

Extracts of a range of higher plants metabolized cystathionine predominantly by β-cleavage of homocysteine, pyruvate and ammonia. γ-Cleavage of cystathionine (to cysteine, α-ketobutyrate and ammonia) could not be demonstrated in the same extracts, and, if present, was less than 0.5% as active as β-cleavage. It follows that transsulfuration in higher plants resembles that in bacteria in proceeding predominantly, perhaps exclusively, in the direction cysteine → (cystathionine) → homocysteine. The physiological significance of the transsulfuration pathway in the de novo synthesis of homocysteine and methionine in higher plants is discussed. β-Cystathionase was purified over 400-fold from spinach leaves and some of its properties determined. The most active substrates are cystathionine, djenkolate and the mixed disulfide of cysteine and homocysteine. The effect of structure on the substrate activity is discussed, and a possible molecular conformation of active substrates is presented. The enzyme is competitively inhibited by β-cyanoalanine, and irreversibly inhibited by 5,5′-dithiobis-(2-nitrobenzoic acid) (DTNB) and N- ethylmaleimide . The substrate specificity of the plant enzyme resembles that of the bacterial enzyme, while the inhibitor sensitivity resembles that of the fungal enzyme.