The presence of at least two different H-blood-group-related beta-D-gal alpha-2-L-fucosyltransferases in human serum and the genetics of blood group H substances.

Sera from H normal, secretors and nonsecretors (H/-, Se/- and H/-, se/se), as well as from H-deficient secretors (h/h, Se/- or Bombay secretors) contain enzyme(s) for the transfer of L-fucose in the alpha-configuration to the 2-position of suitable beta-D-galactopyranosyl units. Sera from H-deficient nonsecretors (h/h, se/se; i.e., Bombay nonsecretors) are devoid of such beta-D-Gal alpha-2-L-fucosyltransferase(s). In order to study these enzymes, a comparison was made of the kinetic properties of the enzymes present in the sera of H-normal nonsecretors (H/-, se/se) with those of H-deficient secretors (h/h, Se/se) with those of H-deficient secretors (h/h, Se/-). These studies revealed a clear difference between the two sources of enzyme: (1) the apparent Km for GDP-fucose was four times lower with the H-normal nonsecretor serum (0.008 mM) than with the H-deficient secretor serum (0.028 mM); (2) acceptors with a type 1 or type 3 chain proved to be better than acceptors with a type 2 chain or than phenyl-beta-D-galactopyranoside for the enzyme present in the serum of H-deficient secretor individuals. Indeed, the synthetic type 2 compound, betaDGal (1-->4)-3-deoxy-beta-DGlcNAc-1-OCH3, which cannot act as an acceptor of beta DGlcNAc alpha-3/4-L-fucosyltransferases, remained unchanged in the serum of an H-deficient secretor but was a good acceptor in the serum of an H-normal nonsecretor, and (3) the alpha-2-L fucosyltransferease activity of the H-deficient secretor serum was more sensitive to heat inactivation than that of the H-normal nonsecretor serum (t1/2 at 46 degrees C were 10 min and 75 min, respectively). These results show that at least two distinct alpha-2-L-fucosyltransferases are present in human serum. It is concluded that the enzymatic activity found in the H-deficient secretor serum (h/h, Se/-) could be the product of the Se gene and the enzymatic activity found in the H-normal nonsecretor serum (H/-, se/se) could be the product of the H gene. This conclusion correlates well with the finding that H and Se genes are closely linked and might have derived by gene duplication in the course of evolution.