Multiple Forms of Cytochrome c in the Rat

Abstract 1. The heterogeneity of cytochrome c obtained in the monomeric form from different organs of rat was studied. Disc electrophoresis on polyacrylamide gel revealed the presence of four forms in heart muscle, liver, and kidney. In the kidney, the mean percentage distribution was Cy I 86.2%, Cy II 8.0%, Cy III 4.2%, and Cy IV 1.6%. 2. The rate at which radioactive iron (59Fe) is incorporated into cytochrome c of heart and kidney was followed in rats in a metabolic steady state. It was found that the isotope was taken up into Cy I at a rate which was approximately 30 times higher in the kidney than in the heart, which is comparable with the difference in the rate of DNA synthesis and cell turnover in these two organs. Furthermore, it was shown that the first fraction to become labeled was Cy I. Thus a distinct heterogeneity of 59Fe labeling of Cy I to Cy III was observed in the kidney, and the change in the degree of labeling as a function of time revealed the characteristics of a precursor-product relationship. This result supports previous observations indicating that only Cy I represents molecules synthesized by the cells, and that Cy II to Cy III are formed from Cy I by simple conversion in vivo. Based on previous studies on multiple forms of bovine heart cytochrome c, as well as on the conversion of rat kidney Cy I in vitro, it is concluded that the chemical basis for the in vivo conversion is a deamidation of asparagine or glutamine residues, or both. 3. The true life span of cytochrome c in the kidney was calculated to be approximately 80 days when based upon the conversion of Cy I into Cy II to Cy III. This value is in good agreement with the rate of conversion as determined in vitro (t½ = 95 days) at 37° and in an electrolyte solution corresponding to the intracellular fluid. From a functional point of view, however, the life span is even longer since Cy III also is capable of electron transfer in biological systems, even though it is less efficient than Cy I and Cy II. 4. The results are discussed in relation to the more general problems concerning the determination of protein turn-over within mammalian tissues. It is stressed that, when dealing with enzymes, one has to differentiate between a physicochemical and a functional turnover, since the rates of these two processes are not necessarily the same.