ИСТИНА

Subunit I of cytochrome c oxidase (COX) from mitochondria and many bacteria contains a cation binding site located near heme a and facing the P outer aqueous phase. Mitochondrial COX binds reversibly Ca2+ or Na+. In the bacterial oxidase of wild type the site is occupied by tightly bound Ca2+. For a long period the role of Ca/Na site remained obscure Recently we have found that Ca2+ binding to COX stabilizes the reduced state of heme a by increasing its midpoint potential by appr.20 mV. Under the same conditions ferrocyanide-induced respiration of bovine COX is reversibly inhibited by Ca2+ but not by Mg2+ ions. The effect is titrated with the apparent Ki value of 10-6M close to that obtained from a Ca2+-induced red shift of heme a absorbance spectra. Similar Ca2+-induced inhibition was observed with a natural electron donor, cytochrome c, when COX was turning over not too fast (less than 10 s-1) and reproduced on mitochondria isolated from different tissues of rat (liver, kidney, heart and skeletal muscle). The inhibition of mitochondrial respiration by Ca2+ ions appeared to be tissue-specific: liver COX isoform (liver, kidney) was blocked by Ca2+ for about 80% while inhibition of heart COX isoform (heart, skeletal muscle) was less than 60%. Titration of the Ca2+-induced inhibition of rat liver mitochondria carried out in Ca-buffer HEDTA in the presence of uncoupler (CCCP) gave the apparent Ki (0.76 x 10-6)M which was found to be very close to the apparent Kd value of (0.5x10-6)M obtained upon a titration of the Ca2+-induced red shift of heme a absorbance spectra in the same mitochondria. Physiologycal significance of this phenomenon is not completely understand. Ca2+-induced inhibition of the mammalian COX resulted in a decrease of the membrane potential at least in case of the COX liver isoform might prevent mitochondria from overloading by Ca2+ after its accidental emission from sarcoplasmic reticulum.