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Nitration of Tyrosyl Residues in - and _sl-Caseins

Eastern Utilization Research and Development Division, USDA, Philadelphia, Pennsylvania 19118

ABSTRACT

Tyrosyl residues in - and _s1-casein were nitrated with tetranitromethane and its effect on their properties determined.

Nitration of up to four of the eight tyrosines in -casein did not affect its _s1-casein-stabilizing ability; additional nitration progressively decreased the stabilizing ability and this was totally absent after all eight tyrosyl residues were nitrated.

Nitrated _s1-casein retained its calcium sensitivity but was unable to form a protective complex with -casein after nitration of four of its 12 tyrosyl residues. A complete tyrosyl nitration of - and _s1-caseins and of a 1:10 -/_s1-complex was achieved at low molar ratios of tetranitromethane. This suggests that these proteins are devoid of major secondary or tertiary structures.