Biosynthesis of the blood group P k and P 1 antigens by human kidney microsomes

Abstract

On human erythrocytes, the membrane components associated with P k and P 1 blood-group specificity are glycosphingolipids that carry a common terminal α- d-Gal p-(1 → 4)-β- d-Gal unit, the biosynthesis of which is poorly understood. Human kidneys typed for P 1 and P 2 (non-P 1) blood-group specificity have been assayed for (1 → 4)-α- d-galactosyltransferase activity by use of lactosylceramide [β- d-Gal p-(1 → 4)-β- d-Glc p-ceramide] and paragloboside [β- d-Gal pNAc-(1 → 3)-β- d-Gal p-(1 → 4)-β- d-Glc p-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 P 1 antigens. The results demonstrated that the microsomal proteins from P 1 kidneys catalyze the synthesis of P k [α- d-Gal p-(1 → 4)-β- d-Gal p-(1 → 4)-β- d-Glc p-ceramide] and P 1 [α- d-Gal p-(1 → 4)-β- d-Gal p-(1 → 4)-β- d-Glc pNAc-(1 → 3)-β- d-Gal p-ceramide] glycolipids, whereas microsomes from P 2 kidney catalyze the synthesis of the P k glycolipid, but not of the P 1 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 P 1 kidneys. The results suggested that the P k and P 1 glycolipids are synthesized by two distinct enzymes.