The tertiary phase of rennin action on αs- and β-caseins

Summary Casein, whole α s -casein and β-casein were incubated for 3 and 14 h with crystalline rennin, at pH 6·60 and 36 °C, both in phosphate buffer and in milk dialysate. Products obtained from both systems, comprising 30–83% calciumsensitive (Ca s ) components, gave similar patterns on starch gel electrophoresis. Whole casein and whole α s -casein were not so soluble in milk dialysate as in phosphate buffer. No significant differences in composition were observed between the Ca s and the calcium-insensitive (Ca 1 ) products from the same source. The α s1 -component of the Ca s product from rennin-treated whole α s -casein had faster gel mobility in comparison to the α s1 -component in the Ca s product from untreated whole α s -casein. Also, α s1 -casein yielded one faster-moving degradation product, while α s2,3,4 appeared unaltered after 14h. The Ca s product of rennintreated β-casein also had faster mobility than untreated β-casein and yielded one faster degradation product and several minor ones of slower mobility. Arginine was the only N -terminal amino acid found in the Ca s product of both rennin-treated and untreated α s - and β-caseins. The arginine content increased from 3·48 and 4·98 moles/10 5 g to 5·12 and 6·38 moles/10 5 g in the Ca s products from rennin-treated β-and α s -caseins, respectively.