To examine whether lysosomal enzymes play a part in the involution of corpora lutea, cathepsin D was assayed in both the lysosomal fraction ('bound' cathepsin D) and the postlysosomal supernatant fluid ('free' enzyme), by measuring the increment in absorbance at 280 nm caused by acid-soluble material released from haemoglobin. From these data the release ratio (free: bound specific activity) was calculated. In corpora lutea on days 5 and 15 of pregnancy, the values for lysosome-bound specific activity (units of E280/h/mg protein) were 4·76 ± 0·51 and 5·00 ± 0·29 (s.e.m.), and the release ratios were 0·12 ± 0·02 and 0·11 ± 0·01 respectively. Similar values were obtained in newly formed corpora lutea of dioestrous and pro-oestrous rats, but on the day of oestrus the level of cathepsin D bound to lysosomes decreased to 2·66 ± 0·36 and the release ratio increased to 0·36± 0·03. On day 5 of prolactin-induced pseudopregnancy, the activities of cathepsin D in both cellular fractions resembled those of pregnancy. An even higher level of lysosome-bound cathepsin D (7·11 ± 0·47) with a relatively low level of free activity (release ratio 0·18 ± 0·02) was observed in lactating rats (day 7 of lactation), in the persistent corpora lutea of pregnancy. Administration of prostaglandin F2α (PGF2α) on days 4, 5 and 6 of lactation significantly decreased the level of lysosome-bound cathepsin D measured on day 7 in the persistent corpora lutea of pregnancy (3·81 ± 0·24) and increased the release ratio (0·40 ± 0·05). The intracellular distribution of acid phosphatase did not show a consistent relationship with the reproductive state. It appears that a decrease in the amount of lysosome-bound cathepsin D is associated with incipient luteolysis, that prolactin inhibits release of cathepsin D from the lysosomes and that PGF2α counteracts this action of prolactin.
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