Protection by substrates and alpha-lactalbumin against inactivation of galactosyltransferase by iodine monochloride.

Iodine is readily incorporated from ICl into galactosyltransferase with total loss of enzymatic activity. The extent of modification was determined by separation of 125I-labeled proteins on sodium dodecyl sulfate polyacrylamide gel electrophoresis. Tyrosines are the only amino acids modified by iodination of galactosyltransferase, a maximum of five, and alpha-lactalbumin, a maximum of three. The major product of the iodination was 3,5-diiodo-L-tyrosine and the remainder was 3-monoiodo-L-tyrosine. Inactivation of galactosyltransferase was dependent on the degree of modification. Substrates of galactosyltransferase are capable of partial protection against enzymatic inactivation. alpha-Lactalbumin, by itself, was also capable of protecting galactosyltransferase against inactivation. This represents evidence for a specific interaction of galactosyltransferase with alpha-lactalbumin in the absence of carbohydrate. The protection of galactosyltransferase by alpha-lactalbumin was enhanced by the presence of the substrates, Mn2+, N-acetylglucosamine, and UDP. Under conditions of either substrate protection or in the absence of substrates, the inactivation reaction of galactosyltransferase by ICl is apparent third order, apparent first order in galactosyltransferase, and apparent second order in galactosyltransferase, and apparent second order in ICl. The apparent order, with respect to ICl, suggests the involvement of 2 mol of ICl either both at one site or one each at two different sites.