The Carboxymethylation of Bovine 
-Lactalbumin
Francis J. Castellino 1 and Robert L. Hill 1
From the
1 From the Department of Biochemistry, Duke University Medical Center, Durham, North Carolina 27706
The rate and extent of reaction of bovine 
-lactalbumin with iodoacetate has been examined under conditions where only the side chains of methionine 90, histidine 32, histidine 68, and histidine 107 are carboxymethylated extensively. Examination of the amino acid composition of partially carboxymethylated -lactalbumin and the tryptic peptides from carboxymethylated -lactalbumin indicated that methionine 90 was the most reactive residue. Histidine 68 reacted with iodoacetate at a slower rate, but more rapidly than histidine 32. Histidine 107 was carboxymethylated at a rate much slower than the other histidyl residues. The relative rates of reaction of these residues are generally in accord with those which may be expected on consideration of the predicted conformation of -lactalbumin. Carboxymethylation of the histidyl and methionyl residues does not completely destroy the ability of -lactalbumin to stimulate lactose synthesis. Carboxymethylation of methionine 90 has little effect on the activity of -lactalbumin, but there is a progressive loss in activity as the histidyl residues are carboxymethylated. When all three residues were modified, about 40% of the activity of -lactalbumin remained.
Submitted on October 3, 1969
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