Effects of microbial proteinase inhibitors on the degradation of endogenous and internalized proteins by rat yolk sacs

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Abstract
The effects of leupeptin and other microbial proteinase inhibitors were measured in rat yolk sacs on the uptake and degradation of formaldehyde-denatured 125-I-labeled bovine serum albumin as well as on the degradation of 3H-labeled endogenous protein. Leupeptin, at concentrations between 1 and 100 .mu.g/ml, inhibits the degradation of added albumin without affecting pinocytic uptake. Large amounts of undegraded albumin accumulate within the tissue. Removal of leupeptin produces a rapid recovery of the capacity to degrade albumin. Endogenous protein degradation is rapidly inhibited by leupeptin, but to a far lesser extent than the breakdown of albumin. The inhibition is slightly reversed on removal of leupeptin. Degradation of both albumin and endogenous protein in intact yolk sacs is inhibited by the microbial proteinase inhibitors in the order: leupeptin > antipain > chymostatin; elastatinal, pepstatin and bestatin are ineffective. Similar results are found when albumin is incubated in yolk-sac homogenates at pH 4 with the inhibitors. The marked inhibitory effects of leupeptin, antipain and chymostatin suggest that cathepsin B and possibly cathepsin L participate in the degradation of 125I-labeled albumin in yolk sacs. By comparison, the smaller inhibitory effects of the proteinase inhibitors on endogenous protein breakdown imply a minor role of lysosomal cathepsins in this process.