Cystathionine- 3-lyase inhibition

Toxicity Causes neurolathyrism and cystathionuria by inhibition of cystathionine y-lyase (EC 4.4.1.1). Biosynthesis metabolism From serine or cysteine and cyanide catalyzed by 3-cyanoalanine synthase (EC 4.4.1.9). 3-Cyanoalanine hydratase (EC 4.2.1.65) hydrates C. to asparagine. ... 

Cystathionine-/3-lyase has been purified 430-fold from spinach leaves (Giovanelli and Mudd, 1971). The enzyme contains a firmly bound pyridoxal phosphate prosthetic group, and was isolated as the holoenzyme. The substrate specificity of the plant enzyme resembles that of the bacterial enzyme, being most active with cystathionine and djenkolate, and far less active with cystine, 5-methylcysteine, and serine, but differs from that of the fungal enzyme, the activity of which with cysteine, serine, and lanthionine is about the same as that with cystathionine. Conversely, with respect to sensitivity to inhibition by 5, 5 -dithiobis-2-nitrobenzoic acid and A-ethylmaleimide, the plant enzyme resembles the fungal enzyme. 

Cystathionine 3"lyase (EC 4.4.1.8), which catalyzes Reaction (2), is also widely distributed in plants, and has been purified from spinach leaves (Giovanelli and Mudd, 1971). The plant enzyme resembles the bacterial more closely than the fungal enzyme in its substrate specificity, but is more similar to the fungal enzyme, with respect to inhibition by sulfhydryl reagents. 

Rhizobitoxine, an analogue of cystathionine produced by certain strains of Rhizobium japonicum (Owens et al., 1972), is a potent irreversible inhibitor of plant cystathionine 8-lyase both in vivo (Giovanelli et al., 1973) and in vitro. Inhibition of the purified spinach enzyme was of the active-site-directed irreversible type (Giovanelli et al., 1971), and probably involves covalent linkage of a cleavage product of rhizobitoxine to the pyridoxal phosphate prosthetic group of the enzyme. 

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