Substrate specificity of S-adenosylhomocysteinase. Cysteine is a substrate of the plant and mammalian enzymes.

Substrate specificity of S-adenosylhomocysteinases (S-adenosyl-L-homocysteine hydrolase, EC 3.3.1.1) with respect to amino acid has been studied using homogeneous preparations of the enzymes from yellow lupin (Lupinus luteus) seeds and bovine liver. Both enzymes use cysteine, in addition to homocysteine, as a substrate. Homoserine, serine, pinicillamine, reduced glutathione and 2-mercaptoethanol are not substrates. In the presence of cysteine, the reaction of S-adenosylthio-amino acid synthesis is characterized by 20-40-fold lower kcat values (kcat = 0.23 s-1 or 0.11 s-1 in the presence of cysteine and either bovine or lupin enzyme) and 270-250-fold higher Km values (Km for cysteine is 15 mM and 35 mM with bovine and lupin enzyme, respectively) than the reaction in the presence of the normal substrate, homocysteine. In the reverse reaction, S-adenosylcysteine is hydrolyzed by the mammalian enzyme much faster than by the plant one. Specificity (kcat/Km) towards S-adenosylcysteine and S-adenosylhomocysteine is 0.9 M-1 . s-1 and 60 000 M-1 . s-1, respectively, with the plant enzyme and 15.3 M-1 . s-1 and 70 000 M-1 . s-1, respectively, with the mammalian enzyme. With plant enzyme, the reactions with cysteine and homocysteine are not competitive, i.e., cysteine does not inhibit the synthesis of S-adenosylhomocysteine, and homocysteine does not inhibit the synthesis of S-adenosylcysteine. This is consistent with independent binding of cysteine and homocysteine to both enzyme subunits. Using adenosine analogs and the mammalian S-adenosylhomocysteinase we were able to synthesize a number of novel S-adenosylcysteine analogs. These included: S-N6-hydroxyadenosyl-L-cysteine, S-2-aminoadenosyl-L-cysteine, S-nebularyl-L-cysteine, S-3-deazaadenosyl-L-cysteine, S-formycyl-L-cysteine, S-N6-methyladenosyl-L-cysteine and S-N1-oxideadenosyl-L-cysteine.