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Effects of Histidine Modification on Kappa-casein¹

Department of Food Science, University of British Columbia, Vancouver 8, Canada

ABSTRACT

Chemical modifications of histidine by 2-phenyl-l,4-dibromoacetoin (PDA), -N-bromoacetyl-arginine methyl ester (BAA), and diethylpyrocarbonate were carried out to evaluate the role of histidine in the -casein molecules in stabilizing Ca-_sl-caseinate and in its susceptibility to rennin action.

PDA alkylated one of three histidine residues in -casein A2, decreasing its stabilizing ability. Aggregation of PDA-alkylated -casein caused by the cross-linkage formation between -caseins through phenylketone groups may be responsible for this decrease.

BAA alkylated one of three histidines in -casein A2 without affecting the stabilizing ability. Aggregation of this casein was negligible. A fraction with 1.5 alkylated residues was separated from -casein Al by DEAE-cellulose chromatography. No decrease in stabilizing ability was observed for this fraction.

Histidine residues were carbethoxyl-ated with diethylpyrocarbonate after casein A2 was sulfenylsulfonated and acylated with citraconic anhydride to protect SS- and amino groups. Carbethoxylation of all three histidines did not decrease stabilizing ability after unblocking of amino groups. Amino-blocking decreased the stabilizing ability which was restored by unblocking. When SS-groups in -casein were unprotected, the stabilizing ability of carbethoxylated -casein decreased. Aggregation of -casein due to SS-interchange reaction may be a cause for decreasing the stabilizing ability of -casein. Carbethoxylation of histidine residues did not affect rennin hydrolysis but inhibited the subsequent aggregation of -casein.

Histidine residues are not essential for the stabilizing ability of -casein.

¹ Supported by Operating Grant A-3641 from the National Research Council of Canada.