Glycosyltransferases in human blood.I. Galactosyltransferase in human serum and erythrocyte membranes

Human serum and hemoglobin-free erythrocyte membranes were found to contain a galactosyltransferase which catalyzes the transfer of galactose from UDP-galactose to specific large and small molecular weight acceptors. The requirements for enzyme activity were found to be similar for the enzymes from both sources. However, the membrane-bound enzyme depended on a detergent for maximal activity. Mn(++) was an absolute requirement for transfer and uridine nucleoside phosphates were inhibitors. The most effective acceptor for galactose was a glycoprotein containing N-acetylglucosamine residues in the terminal position of its oligosaccharide side chains, N-acetylglucosamine was also an acceptor. While the presence of alpha-lactalbumin in the incubation medium resulted in a significant decrease in the transfer of galactose to N-acetylglucosamine, glucose, which was not an acceptor for galactose in the absence of alpha-lactalbumin, became an excellent acceptor. The serum enzyme catalyzed the transfer of 54 nmoles of galactose per milliliter of serum per hour and its apparent K(m) for UDP-galactose was 7.5 x 10(-6)M. The membrane enzyme had a similar apparent K(m). Using a quantitative assay system the enzyme was found to be present in all individuals studied, regardless of their blood type, secretor status, or sex.

Glycosyltransferases in human blood. II. Study of serum galactosyltransferase and N-acetylgalactosaminyltransferase in patients with liver diseases

Serum galactosyltransferase activity was found to be elevated in patients with alcoholic and other liver disorders but remained at a normal level in patients with a variety of nonhepatic diseases. The properties of the galactosyltransferase in patients with liver disease were compared with those of the enzyme in the serum of normal subjects. The possible presence of inhibitors or activators in the serum was examined. Results indicated that in patients with liver disease, the rise in the serum galactosyltransferase was due to an increase in the level of the enzyme present in normal serum and not due to the appearance of a new enzyme. In the cases examined, the level of the enzyme increased with the deterioration of liver function and declined in a patient recovering from acute alcoholic hepatitis. Another glycosyltransferase, an N-acetylgalactosaminyltransferase, was not elevated in the serum of liver disease patients and, unlike the galactosyltransferase, was not detected in normal liver. The results suggest that the serum galactosyltransferase originates from the liver and that an abnormal rise in the level of this enzyme in serum is due to hepatocellular damage.

Peptide hydrolases in the bruch border and soluble fractions of small intestinal mucosa of rat and man

Peptide hydrolases, catalyzing the hydrolysis of 13 dipeptides and 5 tripeptides into their respective amino acids, were studied in small intestinal mucosa and other tissues, in man and in the rat. Studies on the subcellular distribution of these enzymes showed enzyme activities in both the soluble and brush border fractions of the rat small intestinal mucosa, the former constituting 80-90% and the latter 10-15% of the total activity. Zymogram studies of peptide hydrolases, in both fractions, yielded multiple bands indicating multiple zones of enzyme activity. With most substrates a rather broad range of enzyme activities was observed in the soluble fraction differing only slightly from substrate to substrate, the exception being when L-leucyl-L-proline was used: this latter led to a zymogram pattern which was quite distinct. The synthetic substrates, L-leucyl-beta-naphthylamide and L-leucinamide appeared to be hydrolyzed by two electrophoretically distinct enzymes, different from those hydrolyzing other leucyl-containing peptide substrates. Zymogram patterns of the brush border membrane fraction were quite different from those of the soluble fraction of rat small intestine indicating that enzymes from the two sources may be different. No comparable human data were obtained.Peptide hydrolases in the soluble fractions of various organs from the same species gave similar zymogram patterns, while those from the plasma membrane-bound fractions of different organs in the same species were peculiar to each organ. From these data, it is suggested that peptide hydrolases in the brush border and the soluble fractions of small intestine are distinct enzymes and may play different roles in cellular function.

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.