Tunicata

Ecteinascidin 743 is a trace secondary metabolite isolated from the marine tunicate, Ecteinascidia turbinata. Ecteinacidin 743 a most potent antitumor agent, is currently in Phase II clinical trials in Europe and in the USA. A cell-free extract of Ecteinascidia turbinata was used to investigate the biogenetic origin of the ecteinascidins. Incubation experiments with radiolabeled diketopiperazines indicated that the diketopiperazine of tyrosine is the first committed intermediate in the biosynthesis of ecteinascidins. Phenylalanine diketopiperazine was not transformed into the ecteinascidins indicating that this cyclic dipeptide is not an intermediate in the biosynthesis of ecteinascidins. The diketopiperazine of DOPA was used as a cold carrier demonstrating that the diketopiperazine of tyrosine is oxidized to DOPA diketopiperazine and then further transformed to the ecteinascidins.

The ecteinascidins are a family of marine derived alkaloids with potent antitumor activity. Ecteinascidins 743 and 729 are the prime targets due to their superior antitumor activity. beta-Mercaptopyruvic acid is a proposed intermediate in the biosynthesis of ecteinascidins 743/729. A novel synthesis of beta-mercaptopyruvic acid using an enzymatic system was achieved. This was accomplished by trapping the equilibrium between beta-mercaptopyruvic acid and 2,5-dihydroxy-1,4-dithiane-2,5-dicarboxylate using oxidative deamination of cysteine by amino acid oxidase.

Ecteinascidin is a tetrahydroisoquinoline alkaloid isolated from the colonial ascidian Ecteinascidia turbinata. Ecteinascidin exhibits tremendous activity against P388 murine leukemia, however its yields from the tunicate are as low as 10^-4%. A biosynthetic investigation, using in vivo and in vitro methods, has resulted in the identification of the metabolic precursors of ecteinascidin as tyrosine, DOPA and cysteine. Reactive intermediates such as tyrosine and DOPA diketopiperazines have also been identified. These preliminary experiments set the stage for subsequent protein isolation.