Degradation of peptides and proteins of different sizes by homogenates of human MRC5 lung fibroblasts: Aged cells have a decreased ability to degrade shortened proteins

The degradation of haemoglobin and haemoglobin-derived peptide fragments by homogenates of MRC5 fibroblasts has been investigated. Results show that the smaller fragments were degraded more rapidly than larger substrates at both pH 5.5 and pH 7.5. Only the smallest of the soluble cyanogen bromide peptides ( M r 3500) was degraded at pH 7.5. Degradation at pH 5.5 proceeded more rapidly than that at pH 7.5 for all substrates tested but was more marked with the larger substrates. Homogenates prepared from aged cells degraded puromycyl peptides and, to a lesser extent, cyanogen bromide peptides at a slower rate, at pH 7.5, than those prepared from younger cells. We suggest that cytosolic degradation is less selective and at least one cytosolic proteolytic activity decreases as cells age.