Glycoprotein biosynthesis in small intestine. 3. Enzymatic basis for the difference in the antigenicity of mucins

Rat small intestinal mucosa was examined for ability to produce mucins with human blood group A, B, and H activity. Blood group activity of the mucins was compared to antigenic activity of red blood cells in individual rats and the enzymatic basis for differences was investigated. Red cells in all the rats examined contained human blood group A and B antigens. All rats synthesized intestinal mucins having B and H antigenic activity but 57% failed to produce mucins with blood group A activity (A(-)); the remaining 43% (A(+)) produced A substance. The activities of five glycosyltransferases including alpha(1-->2) fucosyltransferase, the determinant of human secretor status, were measured in the intestine of A(+) and A(-) rats. Four enzymes were the same in both groups, while the fifth, N-acetylgalactosaminyltransferase, was present only in A(+) rats. The specificity of this latter enzyme, as found in the rat, appeared similar to that in humans, since it catalyzed addition of N-acetyl-D-galactosamine only to acceptors which had the H determinant structure. In the presence of the enzyme, A(-) mucin could be converted to A(+) mucin; this was shown both by hemagglutination inhibition and immunoprecipitin studies of the products of incubation of A(-) mucin with UDP-N-acetyl-D-galactosamine and the enzyme. These studies indicate that the difference between A(+) and A(-) rats is due to the apparent absence of N-acetylgalactosaminyltransferase in the intestinal mucosa of A(-) rats. These rats may provide experimental models for studies on the effect of ABO and secretor status on susceptibility to ulceration and carcinogenesis.

N-acetyl-D-galactosaminyltransferase in human serum and erythrocyte membranes

This study demonstrates the presence of an N-acetyl-D-galactosaminyltransferase in human serum and in erythrocyte membranes. This enzyme catalyzes the transfer of N-acetyl-D-galactosamine from UDP-N-acetyl-D-galactosamine to a mucin receptor and 2'-fucosyllactose that have blood group H activity and may be responsible, therefore, for blood group A antigenicity. It was present in the serum of individuals with blood group A or AB but was absent from those with blood group B or O. The activity measured in the erythrocyte membrane was low and did not show clear-cut separation among donors of different blood groups. The specificity of this enzyme in serum was suggested by the ability of 2'-fucosyllactose to act as an acceptor, as well as desialyzed porcine submaxillary mucin, while lactose and desialized fetuin failed to accept N-acetyl-D-galactosamine. The catalytic properties of the N-acetyl-D-galactosaminyltransferase from serum and from erythrocyte membranes were similar.

Metabolism of hydroxy fatty acids in dogs with steatorrhea secondary to experimentally produced intestinal blind loops

Several aspects of the metabolism of hydroxy fatty acids were studied in dogs with steatorrhea resulting from an experimentally produced jejunal blind loop. In these animals hydroxy acids were present in the stool in amounts far above normal. These acids disappeared from the feces during tetracycline administration and after exclusion of the blind loop-both procedures that corrected the steatorrhea apparently by reducing bacterial overgrowth. Hydroxy acids persisted in higher than normal amounts, however, after administration of taurocholic acid, which also corrected the steatorrhea, but by a different mechanism. Both in normal dogs and in those with blind loops, hydroxy acid constituted a higher percentage of total fatty acids in the jejunum. A possible conclusion is that hydroxy fatty acids have an enterohepatic circulation via the portal system. When hydroxy acids were fed to normal dogs, steatorrhea was not produced and absorption in amounts similar to that of unsubstituted stearic acid was observed. Isotopic oleic and linoleic acids were converted to hydroxy acids both in vivo and during in vitro incubation with feces; stearic acid was not. These findings support the idea that hydroxy acids arise by the addition of water across double bonds, this addition being catalyzed by enzymes of intestinal bacteria.