Effects of phlorrhizin and thiol reagents on the galactosyltransferase activity of Golgi membrane vesicles of lactating rat mammary gland.

1. The ability of phlorrhizin to inhibit the galactosylation of glucose was re-examined with Golgi membrane vesicles purified from rat mammary gland, and extended to the galactosylation of several glucose analogues and N-acylglucosamines. 2. The inhibition is ascribed, contrary to previous conclusions, to a general annealing of leaky membranes comprising a minority of the vesicles. 3. Three thiol reagents were able to inhibit the galactosylation of N-acylglucosamines with less, or no, inhibition of galactosylation of glucose. This demonstrates the existence of a Golgi membrane carrier that distinguishes between glucose and N-acylglucosamines.