Wilde C J, Addey C V, Knight C H
Hannah Research Institute, Ayr, U.K.
Biochim Biophys Acta. 1989 Sep 15;992(3):315-9. doi: 10.1016/0304-4165(89)90090-1.
Intracellular degradation of newly synthesised casein was measured by a pulse-chase method in freshly prepared goat mammary explants. After incubation in medium containing L-[5-3H]proline, explants were washed and cultured again in unlabelled medium containing 5 mM proline; at intervals up to 24 h the amount of radiolabel incorporated in casein was measured. Tissue was obtained in week 33 of lactation after goats had been milked incompletely in one gland (the test gland) for 24 weeks; the contra-lateral (control) gland was milked normally. In explants from the control gland, casein was not degraded during or after secretion: L-[5-3H]proline incorporated in casein increased to a maximum value which was maintained through the chase period. For four out of five goats, explants from the test gland showed a decrease in total [3H]casein radiolabel at 0-4 h of the chase, indicating that a proportion of casein was degraded during secretion. Intracellular casein degradation was also observed when control gland explants were cultured in chase medium containing a goat whey fraction known to inhibit casein production and milk secretion (Wilde, C.J. et al., (1987) Biochem. J. 242, 285-288). This suggests that the greater volume of residual milk left by incomplete milking reduced secretory efficiency, rendering casein susceptible to intracellular degradation, and that this occurred through the action of a secreted milk constituent, which acts as a chemical feedback inhibitor of milk secretion.
通过脉冲追踪法在新制备的山羊乳腺外植体中测定新合成酪蛋白的细胞内降解情况。在含有L-[5-³H]脯氨酸的培养基中孵育后，将外植体洗涤并在含有5 mM脯氨酸的未标记培养基中再次培养；在长达24小时的间隔时间内，测量掺入酪蛋白中的放射性标记量。组织取自泌乳第33周的山羊，此前一只腺体（试验腺体）未完全挤奶24周；对侧（对照）腺体正常挤奶。在对照腺体的外植体中，酪蛋白在分泌期间或分泌后未发生降解：掺入酪蛋白中的L-[5-³H]脯氨酸增加到最大值，并在追踪期内保持。在五只山羊中的四只中，试验腺体的外植体在追踪的0-4小时显示总[³H]酪蛋白放射性标记减少，表明一部分酪蛋白在分泌期间被降解。当对照腺体外植体在含有已知可抑制酪蛋白产生和乳汁分泌的山羊乳清组分的追踪培养基中培养时，也观察到了细胞内酪蛋白降解（Wilde, C.J.等人，（1987年）《生物化学杂志》242卷，285-288页）。这表明不完全挤奶留下的大量残留乳汁降低了分泌效率，使酪蛋白易受细胞内降解影响，并且这是通过一种分泌的乳汁成分的作用发生的，该成分作为乳汁分泌的化学反馈抑制剂。
