Biosynthesis of the blood group P^k 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 α-D-Galp-(1 → 4)-β-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)-α-D-galactosyltransferase activity by use of lactosylceramide [β-D-Galp-(1 → 4)-β-D-Glcp-ceramide] and paragloboside [β-D-Galp-(1 →4)-β-D-GlcpNAc-(1 → 3)-β-D-Galp-(1 → 4)-β-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 α- and β-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) [α-D-Galp-(1 → 4)-β-D-Galp-(1 → 4)-β-D-Glcp-ceramide] and P₁ [α-D-Galp-(1 → 4)-β-D-Galp-(1 → 4)-β-D-GlcpNAc-(1 → 3)-β-D-Galp-(1 → 4)-β-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 β-lactoside and methyl β-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.