Mechanism for the biosynthesis of Forssman glycolipid from trihexosylceramide.

The mechanism for the synthesis of Forssman glycolipid (GalNAc-alpha-GalNAc-beta-Gal-Gal-Glc-Cer) from trihexosylceramide (Gal-Gal-Glc-Cer, Hex3Cer) and radioactively labeled UDP-N-acetylgalactosamine catalyzed by particulate beta- and alpha-N-acetylgalactosaminyl-transferase of hamster NIL-2K cells was studied. The radioactivity incorporated into the Forssman glycolipid using a mixture of Hex3Cer and globoside (GalNAc-beta-Gal-Gal-Glc-Cer) as the substrates was greater than that from each substrate singly. When the radioactive Forssman glycolipid from the mixed substrates was hydrolyzed with alpha-N-acetylgalactosaminidase, the ratio of 14C-labeled globoside to the liberated N-acetyl[14C]galactosamine was 0.38, indicating that Hex3Cer could serve as a substrate. From these results, the following hypothesis (baton pass model) is proposed. There exists an enzyme complex consisting of beta- and alpha-transferases in membranes. In this complex, it is not possible to saturate alpha-transferase with exogenous globoside; however, it can utilize a transient globoside synthesized from Hex3Cer. It thus appears that Forssman glycolipid is in part formed from Hex3Cer through this enzyme complex without the accumulation of the intermediate globoside. The abbreviations used are: GalNAc, N-acetylgalactosamine; Cer, ceramide.