Intracellular distribution of cathepsin D in rat corpora lutea in relation to reproductive state and the action of prostaglandin F(2α) and prolactin

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 F(2α) (PGF(2α)) 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 PGF(2α) counteracts this action of prolactin.