The action of rennin on casein The disruption of thek-casein complex

Abstract

SUMMARY. Approximately 30 % of the nitrogen of /c-casein was soluble at pH 4-7 after the protein had been treated with rennin at pH 7 while approximately 10 % was soluble in 12% trichloroacetic acid (TCA). The material soluble in 12% TCA appeared at a slower rate initially than did the nitrogen soluble at pH 4-7 but as the reaction proceeded it was released more rapidly.Treating K-casein with urea, or repeated precipitation of the protein at pH 4-7, caused the formation of material insoluble at pH 7, apparently para-/c-casein. Both treatments appeared to free the same soluble fraction as does rennin acting in low concentration or for a short time.Low concentrations of rennin (0-07 /*g/ml) released only part of the available soluble nitrogen from 2 % solutions of whole casein at pH 7. Heating the reaction mixture appeared to restore the casein complex, the restoration being less complete the longer the reaction had proceeded.It is suggested that (c-casein is not a single protein but a complex, and that the action of rennin is first to open the secondary bonds responsible for the stability of this complex.

Studies on thek-casein complex: I. The release of sialic acid-containing material by rennin

“…Materials and methods /c-Casein was prepared by the procedure described by Beeby & Nitschmann (1963). The preparation contained 14-6% nitrogen and 1*8% sialic acid, calculated as l neuraminic acid.…”

“…The same glycopeptide as that released from whole casein was found by Nitschmann & Beeby (1960) and in that portion of the soluble nitrogen from /c-casein that was not precipitated by 12 % TCA. The amount of nitrogen soluble in 12 % TCA found after rennin had acted on /c-casein was 8-10% of the /c-casein nitrogen (Wake, 1959;Tsugo & Yamauchi, 1960;Beeby & Nitschmann, 1963). However, molecular weights as low as 16000 have been reported for /c-casein (Waugh, 1958).…”

Functionality Changes in Proteins Following Action of Enzymes

“…When rennin acts on isolated κ-casein in the absence of calcium ions, an insoluble aggregate forms (155), but 23-30% of the total nitrogen remains in solution (149,156). This reaction has been characterized as follows:…”

Studies on the Action of Rennin on Casein

“…04 M 2-Mercaptoethanol. (1) whole casein type K-AB (2) whole casein type K-B (3) whole casein type K-A (4) K-casein A (5) para-K-casein A…”