Neutral-alkaline proteolytic activity in rat cardiac muscle cells

Homogenates that were prepared from isolated heart muscle cells were found to have reduced ability to degrade endogenous proteins to free amino acids at alkaline pH in comparison to homogenates of whole hearts. The decrease in alkaline proteolytic activity was the result of loss from the particulate fraction. The heart muscle cell preparations retained considerable proteolytic activity at neutral-alkaline pH as indicated by the hydrolysis of [³H]acetylcasein and [¹⁴C]protein from rat heart to acid soluble products. In both cases, the decrease in activity was associated with the particulate fraction and appeared to be the result of the absence of mast cells. The extractable activity of control hearts and heart muscle cells eluted from DEAE-cellulose columns as two peaks of caseinolytic activity. The first peak eluted with 0.07 to 0.12 m NaCl and included a thiol protease. The second peak eluted with 0.25 m NaCl and was seen only in the presence of 5 mm CaCl₂. The specific activity of the Ca²⁺-dependent protease was reduced 50% in muscle cell preparations as compared to control heart.