Rabbit cardiac immunoreactive cathepsin D content during starvation-induced atrophy

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Abstract
To determine whether the increased activity of cathepsin D observed during starvation-induced cardiac atrophy results from activation of preexisting enzyme or synthesis of new enzyme, a solid-phase double-antibody radioimmunoassay was developed for measurement of immunoreactive cathepsin D in extracts of rabbit myocardium. Cathepsin D activity was significantly increased in the hearts of animals starved for 3, 7 and 14 days (82.6 .+-. 0.8, 87.2 .+-. 3.8 and 95.3 .+-. 3.5 U/g wet wt, respectively) compared to controls (65.5 .+-. 1.4 U/g wet wt; P < 0.001). Immunoreactive cathepsin D was increased to a greater extent (168 .+-. 7, 179 .+-. 16, 200 .+-. 17 and 104 .+-. 5 .mu.g/g wet wt for 3, 7 and 14 day starvation and controls, respectively; P < 0.001) than that expected on the basis of the observed increase in enzyme activity. Sephadex G100 gel filtration of cardiac lysosomal extracts from starved and control animals revealed no evidence of high or low MW forms of cathepsin D. The observed increase in cathepsin D activity during starvation-induced cardiac atrophy apparently is accompanied by an increased synthesis and/or decreased degradation of cathepsin D protein, rather than activation of preexisting enzyme. The lower activity levels observed during starvation possibly result from alterations in the concentrations of endogenous inhibitors or activators of cathepsin D.

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