1) Sulfur amino acid 
Cystathionine is an intermediate in the synthesis of cysteine. It is generated from homocysteine and serine by cystathionine beta synthase. It is cleaved into cysteine and α-ketobutyrate by cystathionine gamma-lyase. An excess in the urine is called cystathioninuria. ...
<chemical> Chemical name: L-Homocysteine, S-(2-amino-2-carboxyethyl)-, (R)- ... (12 Dec 1998) ... 
(sis″tә-thi´o-nēn) a thioester of homocysteine and serine; it serves as an intermediate in the transfer of a sulfur atom from methionine to cysteine.
Type: Term Pronunciation: sis′tă-thī′ō-nēn Definitions: 1. The l-isomer is an intermediate in the conversion of l-methionine to l-cysteine; cleaved by cystathionases.
an amino acid, CHONS, that is an intermediate in the transfer of sulfur from methionine to cysteine.