Protein degradation in rabbit reticulocytes and reticulocyte cell-free preparations: stimulation by adenosine triphosphate and inhibition by potassium ions [proceedings]

Tripeptidyl peptidase II in haemolysates and liver homogenates of various species

Rabbit antibodies against purified tripeptidyl peptidase II (TPP II) of human erythrocytes have been prepared and used in immunoblot analysis. Antibodies affinity-purified against the denatured 135,000-Mr subunit of the human peptidase were shown to cross-react with purified rat liver TPP II, as well as a polypeptide of Mr 135,000 in haemolysates and liver homogenates from several other species. TPP II activity in haemolysates from monkey, hog, horse and rabbit corresponded to the levels found in human erythrocytes, whereas this activity was absent or low in erythrocytes from calf and hen. On immunoblot analysis of the calf haemolysates, the 135,000-Mr polypeptide could not be detected. In contrast, analysis of liver homogenates from these species revealed both TPP activity and immunoreactive polypeptides. Immunoreactive polypeptides with Mr about 105,000 and 84,000 of variable occurrence were also detected. In addition, extracts of Escherichia coli showed no TPP II activity under the conditions of the standard assay, although polypeptides of Mr 135,000 and 40,000 were revealed on immunoblot analysis of this species.

Changes in proteinase and peptidase activities during reticulocyte maturation

The ability of rabbit reticulocytes to degrade puromycin-peptides and aminoethylcysteine-induced aberrant polypeptides decreased during cellular maturation. Cell-free studies indicate that the fall in proteolytic activity is not a consequence of accumulation of proteinase inhibitors or the conversion of all of the abnormal protein into undegradable forms. A decrease in peptidase activity using seven dipeptides and one tripeptide as substrates was also found to accompany reticulocyte maturation. Addition of the aminopeptidase B inhibitor bestatin to reticulocyte extracts did not inhibit the conversion of acid-precipitable puromycin-peptides to acid-soluble products; bestatin did induce the accumulation of very low molecular weight material (possibly di- or tripeptides) within the acid soluble fraction.

Effects of ATP and cell development on the metabolism of high molecular weight aggregates of abnormal proteins in rabbit reticulocytes and cell-free extracts

Abnormal proteins synthesized in rabbit reticulocytes in response to (i) the lysine analogue aminoethylcysteine and (ii) puromycin, form high molecular weight aggregates prior to degradation. Inhibitors of ATP synthesis partially inhibit catabolism of the aminoethylcysteine-induced abnormal protein; degradation of puromycin peptides synthesized after incubation with 25 micrograms/ml puromycin was not inhibited. Catabolism of the analogue-induced high molecular weight aggregate of abnormal protein in cell-free extracts was markedly stimulated by ATP, whereas proteolysis of the aggregated puromycin-peptides was ATP-independent. The ability of the reticulocytes to degrade the puromycin-peptide aggregates was found to decrease with cellular maturity. It is suggested that the energy-dependency for proteolysis is in some way related to the chain length of the abnormal protein synthesized.

ATP-independent proteolysis of globin cyanogen bromide peptides in rabbit reticulocyte cell-free extracts

The catabolism of two rabbit globin cyanogen bromide peptides in cell-free extracts of ATP-depleted rabbit reticulocytes has been studied. Proteolysis of the peptides (3533 and 5957 molecular weight) proceeded rapidly in the absence of ATP, had a pH optimum of approximately 7.8, and was inhibited by omicron-phenanthroline, N-ethylmaleimide, cystamine zinc and cobalt ions, and puromycin peptide high-molecular-weight aggregates. Proteolysis of puromycin peptides was inhibited by the rabbit globin cyanogen bromide peptides. The ability of cell-free extracts of degrade the globin cyanogen bromide peptides decreased with the reticulocyte maturity. Blocking the globin cyanogen bromide peptide amino groups by succinic and maleic anhydride treatment decreased susceptibility to degradation. It is suggested that the globin cyanogen bromide peptides might provide model substrates, replacing puromycin peptides, for the investigation of ATP-independent proteolytic events.

Breakdown of aberrant protein in rabbit reticulocytes decreases with cell age

Rabbit reticulocytes, separated into ten cell age groups by bovine serum albumin density-gradient sedimentation, were pulse-labelled with [14C]leucine in the presence of either puromycin dihydrochloride or aminoethyl-L-cysteine (a lysine analogue). Intracellular proteolysis of both the puromyucinyl-peptides and analogue-containing protein decreased with cell maturity.