Bovine γ-Glutamyltransferases

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Abstract
Highly purified preparations of bovine γ-glutamyltransferase obtained from colostrum, kidney, liver, small intestine, and mammary gland were tested for their immunological properties and binding capacity to concanavalin A (Con A). In double immunodiffusion experiments, the transferases gave rise to a single and completely fused precipitin line against rabbit antiserum obtained by immunization with the colostral enzyme. In addition, the immunological identity was studied by quantitative lmmunoprecipitation experiments in which each transferase at a fixed level was incubated with various concentrations of rabbit antiserum against the colostral and renal enzymes. It was concluded that the bovine transferases bear common antigenic determinants of identical reactivity and specificity. The binding capacities of the transferases to Con A-Sepharose were compared in the absence and presence of various concentrations of α-methyl-D-mannoside. The colostral and mammary gland enzymes exhibited the strongest affinity for the lectin, whereas the renal and small intestinal enzymes had little or no affinity. The behavior of the hepatic enzyme was intermediate. These differences in affinity for the lectin persisted after the removal of terminal sialic acid by neuraminidase treatment. In view of the high Con A affinity of the milk enzyme, it was presumed that the transferase molecules present in both fat globule and skim milk membrane surfaces might serve as one of the major Con A-binding sites. This possibility is supported by the fact that the transferase accounted for about one-third of the protein in the “high affinity” fraction under conditions designed to resolve the membrane proteins into “high” and “low affinity” fractions in Con A-Sepharose affinity chromatography.