When positively charged milk proteins can bind to DNA

The binding of three esterified milk proteins (beta-lactoglobulin, alpha-lactalbumin and beta-casein) to plasmid DNAs at pH 7.1 was followed by agarose-gel electrophoresis. Highly esterified beta-lactoglobulin and a-lactalbumin samples showed DNA-binding capacities comparable to those exhibited by native basic proteins such as lysozymes and histones. All the studied esterified beta-casein samples failed to bind to DNA at the applied pH. Complete retardation of DNA migration on agarose gel was observed at a 1:1 ratio of protein basic groups (Lys + Arg) to DNA phosphate acid groups in the case of highly esterified beta-lactoglobulin, esterified a-lactalbumin and native basic proteins (lysozyme and histone). Binding capacity was dependent on the degree of esterification of the milk proteins. Hydrolysis of esterified milk proteins either suppressed or reduced their DNA-binding capacities according to the degree of hydrolysis and consequently to the average size of the resulting peptides. A prolonged peptic hydrolysis (25\% degree of hydrolysis) completely suppressed DNA-binding capacity probably because of the small sizes of the resulting peptic peptides (< 1 kDa). Treatment with trypsin, which hydrolyzed the esterified proteins into relatively large peptide fragments, reduced the DNA-binding capacity to levels inversely proportional to the degree of hydrolysis. In the range of 2.7-12.3 kb, there was no influence of the DNA size on the binding of esterifted milk proteins. The interactions DNA-esterified milk proteins did not depend on the DNA shape (circular or linear). Circular dichroism spectra of DNA in complex with methylated beta-lactoglobulin were markedly altered as compared to those obtained when DNA was in complex with native beta-lactoglobulin.