DISPOSITION OF FUCOSE IN BRAIN 1

— Labelled fucose administered to rats in vivo was rapidly incorporated into brain glycoproteins, but not into any other brain constituents, including glycolipids and acid mucopolysaccharides. Maximum incorporation of tritium‐labelled fucose into brain glyco‐proteins occurred 3–6 h after intraperitoneal injection in young or adult rats, and the half‐time for the turnover of glycoprotein‐fucose in young rats was approximately 2 weeks. Within 3 h after the administration of either [1‐ 3 H]fucose or fucose generally labelled with tritium, 75 per cent of the total acid‐soluble radioactivity in plasma and brain was found to be volatile, and by 24 h after injection more than 90 per cent of the acid‐soluble radioactivity was volatile. The tritium in labelled fiicose appears to undergo arapid exchange reaction with hydrogen atoms in body water, although the tritium in fucose glycosidically‐ linked to glycoproteins is biologically stable. The rapid disappearance of labelled free fucose from the plasma and tissues of the rat precludes the possibility of any significant degree of reutilization of labelled precursor, and provides support for other data indicating that the turnover of fucose in brain glycoproteins is relatively slow in comparison to that of hexosamine and sialic acid. Activities of α‐L‐fucosidase in rat brain, with pH optima at 40 and 6.0, had essentially the same K m (4 × 10 −4 M and 3.2 × 10 −4 M, respectively) with p ‐nitrophenyl‐α‐L‐fucopyranoside as substrate. Activities of both were competitively inhibited by L‐fucose. However, the K t measured at pH 4 (1.9 × 10 −2 ) was almost ten times greater than that measured at pH 6 (1.5 × 10 −4 ).