Biosynthesis and metabolism of cystathionine in Astragalus pectinatus.

Metabolism of L-[35S]cystathionine, L-[35S]cysteine and L-[35S]homocysteine has been investigated in Astragalus pectinatus. The results indicate that cystathionine undergoes both beta and gamma cleavage to give homocysteine and cysteine. Results also show that cystathionine is synthesized from both cysteine and homocysteine. Furthermore, in addition to the incorporation of 35S into cystathionine, incorporation of 35S from cysteine into methionine and from homocysteine into S-methylcysteine is not only in agreement with the above cystathionine cleavage activities, but also suggests, that transsulfuration in A. pectinatus proceeds in both directions, eg. cysteine leads to cystathionine leads to homocysteine and homocysteine leads to cystathionine leads to cysteine. It is suggested, that the latter reaction may be contributing to the net synthesis of cysteine.