THE EFFECT OF LETHAL DOSES OF X-IRRADIATION ON THE ENZYMATIC ACTIVITY OF MITOCHONDRIAL CYTOCHROMEc

Abstract

The coupling of the tetrazolium salts triphenyltetrazolium chloride and nitro-blue tetrazolium to the electron transport chain in mitochondria of thymus, spleen, liver, kidney, and Ehrlich ascites carcinoma cells has been studied with several substrates. In experiments on succinate–triphenyltetrazolium chloride reductase activity it has been possible to demonstrate a radiation lesion in the electron transport chain of mitochondria from thymus and spleen, but not in those from other tissues. This lesion is evident 4 hours after 25 rad of whole-body irradiation, or earlier with higher doses. It is not prevented by the prior administration of aminoethylisothiouronium bromide, serotonin, vitamin K1, or vitamin E, but is reduced by anoxic conditions.Lower levels of cytochrome c are found in irradiated mitochondria isolated from thymus, and the radiation lesion is believed to be produced by loosening the binding of cytochrome c to the mitochondrial membrane after X-irradiation. Decreased levels of ATP occur in thymus, spleen, and ascites cells following irradiation.

Back to the future: The role of cytochrome c in cell death

“…One of them involves activation of the caspase cascade by the interaction of the released hemoprotein with Apaf-1 and pro-caspase-9 in the cytosol, while the other one is related to the retardation of mitochondrial electron transport and subsequent impairment of ATP production and promotion of the generation of reactive oxygen species. This second pathway is very similar to the one described by Scaife (1966) many years ago.…”

“…Neither did in vitro irradiation of isolated mitochondria cause increased enzyme release (Scaife and Hill, 1963). Thus, it was suggested that the perturbation of mitochondrial electron transfer in radiosensitive tissues was based on a controlled release of cytochrome c from the mitochondria and the appearance of the hemoprotein in the cytosol (Scaife, 1964(Scaife, , 1966. It is interesting to note that irradiation damage to cytochrome c in vitro was appreciably less when it was in the fully reduced state rather than partially oxidized or undergoing alternative oxidationreduction changes.…”

“…It is also of interest that only a modest decrease in ATP levels was observed in thymus, spleen and ascites tumor cells following irradiation (Scaife, 1966). This observation suggested that the loss of functional cytochrome c from the mitochondria was not large enough to markedly depress ATP production by the mitochondria or that this loss could be compensated for by glycolytic phosphorylation systems.…”

Apoptosis induced by microinjection of cytochrome c is caspase-dependent and is inhibited by Bcl-2

“…Thus, it seems possible that when Bcl-2 is overexpressed, it may act, perhaps together with other anti-apoptotic proteins (Bag-1? ), as a transporter for cytochrome c back to mitochondria (Scaife, 1966). The capacity of the Bcl-2 to facilitate this putative re-entry of cytochrome c into mitochondria could simply be overloaded.…”

Mitochondrial Lipid Alterations during Fas- and Radiation-Induced Apoptosis

“…Disruption of the electron transport chain (at site II) has been suggested as one mechanism of cell death in irradiation-induced apoptosis of thymocytes (16). Ceramide, a lipid second-messenger, has also been suggested to target site II of the electron transport chain (17).…”