BIOSYNTHESIS OF THE BLOOD GROUP-PK AND P1 ANTIGENS BY HUMAN KIDNEY MICROSOMES

On human erythrocytes, the membrane components associated with P(k) and P1 blood-group specificity are glycosphingolipids that carry a common terminal alpha-D-Galp-(1 --> 4)-beta-D-Gal unit, the biosynthesis of which is poorly understood. Human kidneys typed for P1 and P2 (non-P1) blood-group specificity have been assayed for (1 --> 4)-alpha-D-galactosyltransferase activity by use of lactosylceraminde [beta-D-Galp-(1 --> 4)-beta-D-Glcp-ceramide] and paragloboside [beta-D-Galp-(1 --> 4)-beta-D-GlcpNAc-(1 --> 3)-beta-D-Galp-(1 --> 4)-beta-D-Glcp-ceramide] as acceptor substrates. The linkage and anomeric configuration of the galactosyl group transferred into the reaction products were established by methylation analysis before and after alpha- and beta-D-galactosidase treatments, as well as by immunostaining using specific monoclonal antibodies directed against the P(k) and P1 antigens. The results demonstrated that the microsomal proteins from P1 kidneys catalyze the synthesis of P(k) [alpha-D-Galp-(1 --> 4)-beta-D-Galp-(1 --> 4)-beta-D-Glcp-ceramide] and P1 [alpha-D-Galp-(1 --> 4)-beta-D-Galp-(1 --> 4)-beta-D-GlcpNAc-(1 --> 3)-beta-D-Galp-(1 --> 4)-beta-D-Glcp-ceramide] glycolipids, whereas microsomes from P2 kidney catalyze the synthesis of the P(k) glycolipid, but not of the P1 glycolipid. Competition studies using a mixture of two oligosaccharides (methyl beta-lactoside and methyl beta-lacto-N-neotetraoside) or of two glycolipids (lactosylceramide and paragloboside) as acceptors indicated that these substrates do not compete for the same enzyme in the microsomal preparation from P1 kidneys. The results suggested that the P(k) and P1 glycolipids are synthesized by two distinct enzymes.