gamma-Glutamyl transpeptidase: relation to immunoglobulin A and free secretory component.

The experiments reported show that bovine γ-glutamyl transpeptidase can be separated from free secretory component. An ion-exchange Chromatographic procedure was developed to analyze the incubation mixtures of the enzyme with glutathione or S-(2-acetamido)-glutathione and glycylglycine. Using this system or the γ-glutamyl p-nitroanilide assay, no significant transpeptidase activity could be detected in the free secretory component-containing fractions of DEAE-cellulose chromatography. Gel filtration on Biogel A-5M showed that the bovine whey transpeptidase chromatographed in the void volume suggesting an aggregate of a minimum molecular weight of about 5 × 10⁶. The transpeptidase could be separated from all immunoglobulins in bovine whey and human colostrum by a combination of agarose gel filtration and immunoadsorption. Concentrated samples of human and sheep saliva showed normal amounts of secretory component, but no detectable γ-glutamyl transpeptidase activity. These experiments show that (1) the transpeptidase and secretory component are two different proteins, and (2) the transpeptidase is present in bovine and human milk as a high molecular weight aggregate which does not include any of the immunoglobulins.