J. Biol. Chem., Vol. 255, Issue 23, 11262-11267, 12, 1980
Protection by substrates and alpha-lactalbumin against inactivation of galactosyltransferase by iodine monochloride
JS Silvia and KE Ebner
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.
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