Spectral evidence of multiple cytochromes b present in succinate: cytochrome c reductase

Phosphophloretin sensitivity of rabbit renal NaPi-IIa and NaPi-Ia

The effect of phosphorylated phloretins on Na+-dependent phosphate uptake into rabbit renal brush-border membrane vesicles (BBMV) was examined. Na+-dependent phosphate uptake into isolated rabbit cortex BBMV was sensitive to 2′-phosphophloretin (2′-PP) and 2′-phospho-4′,4,6′-trimethoxy phloretin (PTMP) in a dose-dependent and pH-dependent manner. PTMP inhibition of Na+-dependent phosphate uptake was maximum at alkali pH, and 2′-PP inhibition of Na+-dependent phosphate uptake was maximum at acidic pH. Increasing Na+concentrations did not increase PTMP inhibition of renal cortex BBMV Na+-dependent phosphate uptake at pH 6. The effect of phosphophloretins on Na+-dependent phosphate uptake was examined in BBMV isolated from purified proximal tubules and distal tubules. 2′-PP and PTMP inhibition of Na+-dependent phosphate uptake into BBMV isolated from purified proximal tubules was similar to the inhibition seen with BBMV from renal cortex. 2′-PP, but not PTMP, inhibited Na+-dependent phosphate uptake into BBMV isolated from purified distal tubules. The pH dependence of inhibition, the absence of PTMP inhibition of Na+-dependent phosphate uptake into distal tubule BBMV, and the inhibition of Na+-dependent phosphate uptake into distal tubule BBMV suggest that NaPi-Ia is 2′-PP sensitive and NaPi-IIa is PTMP sensitive.

Oxidation-reduction potentials and spectral properties of some cytochromes from Thiobacillus versutus (A2)

Cytochromes c-550 (acidic), c-550 (basic), c-551 and c-552.5 from Thiobacillus versutus have been highly purified and characterized. Their spectral properties at 77 K are described. Oxidation-reduction titrations of cytochromes c-550 (acidic) and c-550 (basic) showed them to exhibit Nernst values of n = 1, with single redox centres in the cytochromes, and to have midpoint redox potentials at pH 7.0 (Em,7) of 290 and 260 mV, respectively. Cytochrome c-551 contained two separately titratable redox components, each giving n = 1. The low potential centre (55% of titratable cytochrome) and the high potential centre (45%) had Em,7 values of -115 and +240 mV, respectively. Cytochrome c-552.5 also contained at least two redox centres. One (65% of titratable cytochrome) had n = 1 and Em,7 = 220 mV. The remaining 35% appeared to be a low potential component with an Em,7 possibly as low as -215 mV. the roles of these cytochromes in respiratory thiosulphate oxidation are discussed.

Three functionally different cytochrome b redox centres in pigeon heart mitochondria

Three functionally different cytochrome b redox centres, apparently of high metabolic activity, were detected in intact pigeon heart mitochondria; cytochrome b(1), b(m) and b(h), with maxima of absorption at 556.6 (State 5), 560.6, and 564.5 nm, respectively (alpha-bands, 77K). 2. Cytochrome (b(l) was reduced in the presence of either antimycin or HQNO (2-heptyl-4-hydroxyquinoline N-oxide). The absorption maximum was shifted by dithionite, cyanide, NNN'N'-tetramethyl-p-phenylenediamine + ascorbate, HQNO and antimycin. The spectra obtained on simultaneous or successive addition of HQNO and antimycin favoured the assumption of a common binding site for the two inhibitors. 3. Cytochrome b(m) was reduced in the presence of HQNO, but not in the presence of antimycin. No shifts of absorption maximum was observed. 4. Cytochrome b(h) was reduced in the presence of antimycin. HQNO was unable to cause reduction of this cytochrome by endogenous substrates. The absorption maximum was shifted to lower wavelength by organic solvents. It was inseparable from that of cytochrome b(m) in the presence of 0.4% ethanol. 5. The pattern of reduction in the presence of HQNO or antimycin demonstrates the functional difference of the three redox centres and appears incompatible wih a linear respiratory chain.

Cytochrome b-565 in Saccharomyces cerevisiae: use of mutants in the cob-box region of the mitochondrial DNA to study the functional role of this spectral species of cytochrome b. 1. Measurements of cytochromes b-562 and b-565 and selection of revertants devoid of cytochrome b-565

In order to study the functional role of the spectral species of cytochrome b-565 observed in mitochondria, genetically manipulated strains of Saccharomyces cerevisiae have been used. Strains have been found which are devoid of cytochrome b-565 under certain conditions and which nevertheless are able to grow on a respirable substrate. Two different methods have been used to determine the cytochrome b-565 content: anaerobic titrations and antimycin-A-induced reduction of cytochrome b-565. Both yield the same results.

Stepwise reduction of cytochromes b-562, b-566 and b-558 in rat liver mitochondria

1. Addition of KCN to aerobic, rotenone-inhibited rat liver mitochondria with out addition of substrate caused reduction of cytochromes b-562 (having an alpha-band at 562 nm at room temperature), c + c1, and a + a3. The effect of KCN on cytochrome b-562 was reversed by pentachlorophenol, though the effect of KCN on cytochromes c+c1 and a+a3 was not reversed by this uncoupler.2. Addition of ATP to aerobic, rat liver mitochondria inhibited with 500 muM KCN under conditions were cytochromes b-562, c+c1 and a+a3 were reduced, caused reduction of cytochrome b-566. The absorbance spectrum of cytochrome b-566 had an alpha-band at 565.5 nm, a beta-band at 538 nm and a gamma-band at 431 nm, but no shoulder around 558 nm at room temperature. 3. Addition of succinate to rotenone-KCN-inhibited and ATP-treated rat liver mitochondria under conditions where cytochromes b-566, b-562, c+c1 and a+a3 were already fully reduced, caused reduction of cytochrome b-558 (having an alpha-band at 558 nm, a beta-band at 527 nm and a gamma-band at 426 nm at room temperature) after exhaustion of molecular oxygen in the reaction medium, without any contribution from a long-wavelength species (cytochrome b-566). 4. It was concluded that the 558-nm band is not a short-wavelength shoulder of cytochrome b-566, but is due to a different species from cytochrome b-566.

Multiple forms of cytochrome b in Mycobacterium phlei: kinetics of reduction

The kinetics of reduction of the b-type cytochromes in the electron transport particles (ETP) from Mycobacterium phlei were studied with nicotinamide adenine dinucleotide, reduced form (NADH) or succinate as electron donors. There appeared to be three active cytochromes b in the ETP,bS563 and bS559, which were reducible by either substrate, and bN563, which was reducible by NADH but not by succinate. In the presence of adenosine 5'-triphosphate, a substantial increase in b563 reduction was observed with succinate at anaerobiosis. This was followed by a decrease in absorption. Adenosine 5'-triphosphate did not effect an increase in cytochrome b563 reduction at transition with NADH, but the occurrence of a secondary decrease in absorption was reflected in a decrease in total enzymatic reduction. The adenosine 5'-triphosphate effect was altered in trypsin-treated ETP, and abolished by uncoupling agents or by removal of the coupling factor-latent adenosine triphosphatase. In the presence of a supernatant fraction obtained during the preparation of the ETP, b563 reduction with succinate was greatly increased. A smaller increase was observed with NADH. Cytochrome b reduction was also studied in ETP inhibited by 2-n-nonylhydroxyquinoline-N-oxide, which appears to inhibit at bS563. On the basis of these data the interrelationships among the b-type cytochromes can be described in relation to the M. phlei electron transport chain.