The binding of cytochrome b5 to plasma membranes of rat liver. Its implication for membrane specificity and biogenesis

The in vitro incorporation of cytochrome $\text{b}{}_5$ into purified plasma membranes was investigated by biochemical and immunological methods. Plasma membrane preparations incorporated three times less cytochrome $\text{b}{}_5$ than did microsomal preparations; 60% of this cytochrome $\text{b}{}_5$ could not be reduced by the NADH-cytochrome $\text{b}{}_5$ reductase and was considered as being bound to the plasma membrane. The morphological observations made after the immunochemical labeling of cytochrome $\text{b}{}_5$ clearly showed a good but asymmetrical distribution of the ferritin labeling: only the inner face of the plasma membrane incorporated cytochrome $\text{b}{}_5$. These results are discussed with respect to theories which concern the subcellular membrane relationships in the cell.     

A new assay for ATP sulfurylase based on differential solubility of the sodium salts of adenosine 5′-phosphosulfate and sulfate

A new assay for ATP sulfurylase (ATP:sulfate adenylyltransferase, EC 2.7.7.4) has been devised, the key feature of which is the quantitative precipitation of the unreacted substrate as Na₂SO₄, in ethanol:water (5:1), leaving the product adenosine 5′-phosphosulfate (APS) in solution with an 80% yield. This separation procedure, coupled with the use of $\text{[}{}^{35}\text{S]SO}{}_4{}^{\mathrm{2?}}$, makes it possible to assay the synthesis of picomole amounts of [³⁵S]APS and thereby determine accurately the initial velocity of the forward reaction catalyzed by ATP sulfurylase. Extracts from tobacco cells synthesized [³⁵S]APS and a negligible amount of one unknown sulfur compond, but they did not synthesize 3′-phosphoadenosine 5′-[³⁵S]phosphosulfate (PAPS) under the assay conditions adopted, so that the ³⁵S remaining in solution after ethanol precipitation of $\text{[}{}^{35}\text{S]SO}{}_4{}^{\mathrm{2?}}$ is a valid assay of the ATP sulfurylase reaction in these extracts. This assay is simple, rapid, and suitable for assaying multiple samples. Extracts of four additional species of plants were incubated under the same conditions, and each catalyzed the synthesis of [³⁵S]APS according to this assay. The assay is suitable for use with the enzyme from any organism, provided that the product APS is not acted upon by another enzyme.     

Sensitive substrates for human leukocyte and porcine pancreatic elastase: a study of the merits of various chromophoric and fluorogenic leaving groups in assays for serine proteases.

Five sensitive substrates of human leukocyte and porcine pancreatic elastase having the sequence MeO-Suc-Ala-Ala-Pro-Val-X where -X is -NA (4-nitroanilide), -SBzl (thiobenzyl ester), -OEt, -AMC (4-methyl-7-coumarylamide), or -NNapOMe (1-methoxy-3-naphthylamide), were synthesized. The kinetic constants for the enzymatic hydrolysis as well as the sensitivity of each substrate are reported. Hydrolysis of the peptide -AMC and -NNapOMe derivatives were followed by monitoring spectrofluorometrically the release of H-AMC and H-NNapOMe, respectively. Cleavage of the thiobenzyl ester yields benzyl mercaptan as the hydrolysis product. Its release was monitored at 412 nm by reaction with Ellman's reagent [5,5′-dithiobis(2-nitrobenzoic acid)] in the assay mixture to produce the 3-carboxy-4-nitrothiophenoxide anion or at 324 nm by reaction with 4,4′-dithiodipyridine to produce 4-thiopyridone. Hydrolysis of the ethyl ester was measured using a coupled assay with NAD⁺ and liver alcohol dehydrogenase. The NADH⁺ formed upon oxidation of the ethanol released from cleavage of the ester was followed at 340 nm. MeO-Suc-Ala-Ala-Pro-Val-SBzl, the best substrate of the series, was capable of detecting as little as 2.4 pm (0.072 ng/ml) of active-site titrated human leukocyte elastase and 5.8 pm (0.15 ng/ml) of active-site titrated porcine pancreatic elastase using 4,4′-dithiodipyridine. The corresponding values with Ellman's reagent were 5.0 pm (0.15 ng/ml) and 7.4 pm (0.19 ng/ml), respectively. Advantages of this substrate are its high $\text{k}{}_{\mathrm{<mtext>cat</mtext>}}\text{K}{}_{\mathrm{m}}$ values and ease of synthesis. One disadvantage is the interference of high concentration of thiols with the assay. The peptidyl-AMC is almost as sensitive as the thiobenzyl ester and can detect 11 pm of HL elastase and 18 pm of PP elastase. An advantage of this substrate is the fact that cleavage involves a peptide bond. Disadvantages are relatively low $\text{k}{}_{\mathrm{<mtext>cat</mtext>}}\text{K}{}_{\mathrm{m}}$ values and greater difficulty in synthesis. The peptidyl-NNapOMe has possible utility in histochemical studies due to the low intrinsic fluorescence of the substrate relative to the peptidyl-AMC. For a rate assay it has a lower sensitivity than either the thiobenzyl ester or peptidyl-AMC. The coupled liver alcohol dehydrogenase-ethyl ester assay offers no advantages except in cases where the ester substrate is commercially available. The 4-nitroanilide assay enjoys moderate sensitivity and is extremely convenient for routine use. Except for the peptidyl ethyl ester assay, all of the human leukocyte elastase assays reported in this paper are vastly more sensitive than any other existing assay for this protease.     

Cytoplasmic poly(A) polymerase from sea urchin eggs, merogons, and embryos

The presence of cytoplasmic poly(A) polymerase has been established in sea urchin eggs and four-cell embryos by subcellular fractionation and use of enucleate egg halves. ATP is the only ribonucleoside triphosphate incorporated. This incorporation is time dependent, contingent on input protein concentration, and immune to a variety of antimetabolites known to inhibit DNA-directed RNA synthesis. Both the unfertilized egg and the four-cell embryo cytoplasmic poly(A) polymerase activities display a preference for Mn²⁺. While oligo(A)₄ is inactive as a primer, addition of $\text{oligo(A)}{}_{\mathrm{<mtext>16</mtext>}}\text{,}\text{poly(A)}{}_{\mathrm{<mtext>45</mtext>}}$ and $\text{poly(A)}{}_{\mathrm{<mtext>90</mtext>}}$ stimulates ATP incorporation. On a unit per milligram protein basis, the endogenous activity associated with cytoplasmic fractions obtained from nucleate and enucleate egg halves is 36 and 83% that obtained with the cytoplasmic fraction prepared from the unfertilized egg. In the presence of $\text{oligo(A)}{}_{\mathrm{<mtext>16</mtext>}}$, both the nucleate and enucleate egg halves exhibit 81% of the activity associated with the unfertilized egg cytoplasmic fraction. The level of Mn²⁺ cytoplasmic poly(A) polymerase activity from the four-cell embryo is approximately 50% that of the unfertilized egg. This decrease does not appear to be due to either a postfertilization alteration in the subcellular localization of poly(A) polymerase or an increase in RNase activity. Supplementation with $\text{oligo(A)}{}_{\mathrm{<mtext>16</mtext>}}$ failed to restore the four-cell embryo cytoplasmic poly(A) polymerase potential to a level comparable to that of the unfertilized egg. Suppression of postfertilization protein synthesis by emetine, however, prevents this developmental decline in ATP incorporation thereby suggesting that postfertilization cytoplasmic poly(A) polymerase activity is subject to negative translational control.     

Binding of bovine cytochrome b5 to phosphatidycholine liposomes Characterization of the reconstituted lipid-protein vesicles

Cytochrome $\text{b}{}_5$ was extracted and purified from beef liver by a detergent method (cytochrome $\text{d-b}{}_5$). The hydrophilic moiety which carries the heme group (cytochrome $\text{t-b}{}_5$) was prepared by trypsin action upon pure cytochrome $\text{d-b}{}_5$.Single-shelled lecithin liposomes form complexes with cytochromes $\text{d-b}{}_5$ up to a molar ratio of one protein for 35 phospholipids. The lipid-protein complexes were isolated by gel filtration on Sepharose 4B. They are hollow vesicles in which [³H]-glucose can be trapped. Their diameter is greater than that of the initial liposomes.Cytochrome $\text{t-b}{}_5$ does not interact with the vesicles. These results show that the hydrophobic tail is necessary for the binding and that the hydrophilic part of the protein is located on the outer face of the vesicles. This asymmetry is also proved by the action of reducing agents.Experiments with saturated phosphatidylcholines show that the protein interacts with the lipids both below the transition temperature $\text{T}{}_{\mathrm{M}}$. i.e. when the aliphatic chains are in a crystalline state, and above $\text{T}{}_{\mathrm{M}}$, when the alipathic chain are in a fluid state.¹H NMR spectra show that even at the maximum cytochrome $\text{d-b}{}_5$ concentration the presence of the proteins does not markedly change the dynamics to the phospholipid molecules. An asymmetric single-shelled vesicle structure is proposed for the complex.     

Bicarbonate ion-ATPase in rat liver cell fractions

The distribution of $\text{HCO}{}_3{}^{\mathrm{?}}\text{-}\text{ATPase}$ activity was studied in cell fractions prepared from homogenates of rat liver. The level of mitochondrial contamination in the microsomal fraction depended on the fractionation procedure and on the method of homogenization. With proper care, microsomes with undetectable mitochondrial contamination could be prepared. These microsomes had no detectable $\text{HCO}{}_3{}^{\mathrm{?}}\text{-ATPase}$ activity. Approximately 85 % of the total $\text{HCO}{}_3{}^{\mathrm{?}}\text{-ATPase}$ activity of the post 6000 x g · min supernatant was recovered in the mitochondrialfraction. The properties of this mitochondrial $\text{HCO}{}_3{}^{\mathrm{?}}\text{-}\text{ATPase}$ were not distinguishable from those of the various microsomal $\text{HCO}{}_3{}^{\mathrm{?}}\text{-}\text{ATPase}$ previously described by other investigators.     

An adaptation of the sodium periodate/sodium borotritide labeling procedure which allows the unambiguous identification of synaptic sialoglycoproteins

An adaptation of the sodium periodate/sodium borotritide procedure for the identification of membrane sialoglycoproteins is described which eliminates interference from nonspecifically incorporated tritium. Synaptic membranes were labeled using the $\text{NaIO}{}_4\text{NaB}{}^3\text{H}{}_4$ procedure and separated by polyacrylamide gel electrophoresis. Following electrophoresis the gels were fixed, sliced, and individual slices treated with neuraminidase. Treatment with neuraminidase selectively released [³H]sialyl derivatives from the fixed glycoproteins allowing the unambiguous identification of sialoglycoproteins. The sialoglycoprotein composition of synaptic membranes and synaptic junctions was compared.     

Identification and partial characterization of plasma membrane polypeptides of Trypanosoma brucei

A plasma membrane-enriched vesicle fraction has been prepared from Trypanosoma brucei by sonication and differential centrifugation on sucrose gradients. This fraction is enriched 5-fold in the plasma membrane marker enzymes adenyl cyclase (EC 4.6.1.1) and ouabain-inhibitable, ($\text{Na}{}^{\mathrm{+}}\text{+}\text{K}{}^{\mathrm{+}}$)-dependent adenosine triphosphatase (EC 3.6.1.3). It is also enriched up to 14-fold in iodinated surface proteins, and up to 4-fold in [³H]mannose-labeled glycoproteins, of which the major variable surface coat glycoprotein is the main constituent. Proteins of the plasma membrane fraction and other subcellular fractions have been identified by electrophoretic analysis in sodium dodecyl sulfate-polyacrylamide gradient slab gels. Several high molecular weight surface glycopeptides have been selectively investigated and partially characterized by a combination of metabolic labeling with [³H]mannose, lactoperoxidase-catalyzed surface iodination, and affinity chromatography on Con A-Sepharose. In addition to the major variable surface coat glycoprotein (estimated $\text{M}{}_{\mathrm{<mtext>r</mtext>}}\text{= 58 000}$), there are several minor surface glycopeptides ($\text{M}{}_{\mathrm{<mtext>r</mtext>}}\text{= 76 000, 86 000}\text{and}\text{92 000–100 000}$) which are apparent extrinsic membrane components, and two surface glycopeptides ($\text{M}{}_{\mathrm{<mtext>r</mtext>}}\text{= 42 000}\text{and}\text{130 000}$) which are intrinsic membrane components.     

Purification of bovine liver microsomal NADH-cytochrome b5 reductase using affinity chromatography

Microsomal NADH-cytochrome $\text{b}{}_5$ reductase has been purified from bovine liver by an improved procedure which employs affinity chromatography on ADP-agarose in combination with anion exchange chromatography. The reductase was extracted from a 105,000 × $\text{g}$ microsomal pellet with Triton X-100. The overall purification from isolated microsomes was 98-fold and the yield was 10%. The preparation was nearly homogeneous on SDS-PAGE. This procedure requires less time and effort than previously described procedures. Partially purified cytochrome $\text{b}{}_5$ is also obtained.     

Ligand-dependent tryptic inactivation of the ouabain sensitivity of ADP-ATP exchange catalyzed by canine renal Na+, K+-ATPase.

Phosphate transporter of bovine heart mitochondria was purified by solubilization of submitochondrial particles with octylglucoside and fractionation of the extract with ammonium sulfate. After reconstitution into liposomes the purified protein catalyzed phosphate transport which was sensitive to mersalyl and other SH reagents. Transport measured either as $\text{P}{}_{\mathrm{i}}\text{OH}$ or $\text{P}{}_{\mathrm{i}}\text{P}{}_{\mathrm{i}}$ exchange was proportional to protein concentration and time. The $\text{P}{}_{\mathrm{i}}\text{OH}$ but not the $\text{P}{}_{\mathrm{i}}\text{P}{}_{\mathrm{i}}$ exchange was stimulated several fold by valinomycin plus nigericin in the presence of K⁺. The reconstituted system provides a suitable assay during purification of the mitochondrial phosphate transporter.     

A radioimmunoassay for copper containing superoxide dimutase.

Highly radioactive rat and bovine copper containing superoxide dismutases (Cu-SOD) up to 1700 Ci/mmole of protein were prepared by iodination with ¹²⁵I and chloramine T. Labelled human Cu-SOD was obtained by coupling the enzyme with N-succinimidyl 3-(4-hydroxy, 5-(¹²⁵I)-iodophenyl) propionate. These tracer molecules can be used with anti-bovine or anti-human Cu-SOD antisera to estimate various Cu-SODs as protein (rat, bovine, human) with a sensitivity of 20–80 picogrammes, using standard radio-immunochemical techniques.Control in vivo of the steady state concentration of the radical ion superoxide $\text{O}{}_2{}^{\mathrm{<mtext>?</mtext>}}$ strongly depends upon the quantity of superoxide dismutase (SOD) present (1). There are several types of enzyme, which are ubiquitous among all aerobic species, but differ in their subcellular location and in the nature of the metal at the active site (2).It is thus of interest to develop a sensitive, specific, and easy assay of the enzymes, particularly if discrimination between mangano and cuprozinc SODs can be achieved. Radioimmunological techniques answer these requirements and can contribute to the solution of a wide variety of problems connected with the metabolism of molecular oxygen.     

Immunochemical study on the participation of cytochrome b5 in drug oxidation reactions of mouse liver microsomes.

Highly purified divalent and monovalent antibodies against cytochrome $\text{b}{}_5$, anti-$\text{b}{}_5$ immunoglobulin G (IG) and anti-$\text{b}{}_5$ Fab', were used in elucidating the role of this cytochrome in the drug-oxidizing enzyme system of mouse liver microsomes. Anti-$\text{b}{}_5$ IG strongly inhibited not only NADH-supported but also NADPH-supported oxidation of 7-ethoxycoumarin and benzo(a)pyrene, but had no inhibitory action on the oxidation of aniline. Anti-$\text{b}{}_5$ Fab' also inhibited NADH-supported and NADPH-supported benzo(a)pyrene hydroxylation. These observations indicate an essential role of cytochrome $\text{b}{}_5$ in the transfer of electrons not only from NADH but also from NADPH to cytochrome P-450 in the microsomal oxidation of some drugs, but not of aniline.     

Further studies on the effect of aldosterone on electrical resistance of toad bladder

The fall in transepithelial electrical resistance which accompanies aldosterone stimulation of short-circuit current ($\text{I}{}_{\mathrm{<mtext>sc</mtext>}}$) in toad urinary bladder has been studied further to evaluate the possible causal role of this response in hormonal stimulation of Na⁺ transport. A steady-state change in tissue conductance was found to depend upon both the simultaneous stimulation of transport by the steroid and the metabolic state of the tissue. Changes in metabolic state alone did not alter resistance. A sustained increase in Na⁺ transport, dependent on pretreatment with aldosterone and elicited by addition of glucose, could be obtained without a sustained decrease in resistance. Amiloride, an inhibitor of Na⁺ uptake, produced changes in $\text{I}{}_{\mathrm{<mtext>sc</mtext>}}$ that were linearly correlated with its effects on tissue conductance. On the basis of the $\text{conductance}\text{-I}{}_{\mathrm{<mtext>sc</mtext>}}$ relationship with amiloride, the $\text{I}{}_{\mathrm{<mtext>sc</mtext>}}$ response to aldosterone was about two-fold higher than would be predicted from its effects on conductance alone. Despite the apparent lack of a simple quantitative dependence of the change in $\text{I}{}_{\mathrm{<mtext>sc</mtext>}}$ on the change in conductance when the response is fully developed, the results suggest that conductance changes may mediate the initial or early stage of the response.     

Lipid-protein interactions in membrane models fluorescence polarization study of cytochrome b5-phospholipids complexes

According to previous authors, cytochrome $\text{b}{}_5$, when extracted from bovine liver by a detergent method, is called cytochrome $\text{d-b}{}_5$. On the other hand, the protein obtained after trypsin action, which eliminates an hydrophobic peptide of about 54 residues, is called cytochrome $\text{t-b}{}_5$.Fluorescence polarization of the dansyl phosphatidylethanolamine probe inserted into phospholipid vesicles is very senstive to the binding of proteins, and so is a useful method to study lipid-protein interactions.The chromophore mobility, $\text{R}$, decreases markedly when dipalmitoyl phosphatidylcholine vesicles are incubated with cytochrome $\text{d-b}{}_5$, whereas $\text{R}$ does not change for cytochrome $\text{c}$ and cytochrome $\text{t-b}{}_5$. This can be interpreted as a strengthening of the bilayer, only due to the interaction of the hydrophobic peptide tail.Interaction of dipalmitoyl phosphatidylcholine vesicles with cytochrome $\text{d-b}{}_5$ occurs either below or above the melting temperature of the aliphatic chains (41 °C). Even for a high protein to lipid molar ratio (1 molecule of protein for 40 phospholipid molecules), the melting temperature is apparently unaffected.Phosphatidylserine and phosphatidylinositol do not interact at pH 7.7 with cytochrome $\text{d-b}{}_5$, because electrostatic forces prevent formation of complexes. At low pH, the interaction with the protein occurs, but the binding is mainly of electrostatic nature.     

Heterozygote frequencies in small subpopulations.

The mean fixation index within subpopulations (F_IS) has been defined as $\text{F}\text{̄}{}_{\mathrm{IS}}\text{= ∑w}{}_{\mathrm{i}}\text{F}{}_{\mathrm{ISi}}\text{or as}\text{F}\text{̂}{}_{\mathrm{IS}}\text{=}\text{∑w}{}_{\mathrm{i}}\text{p}{}_{\mathrm{i}}\text{q}{}_{\mathrm{i}}\text{F}{}_{\mathrm{ISi}}\text{∑w}{}_{\mathrm{i}}\text{p}{}_{\mathrm{i}}\text{q}{}_{\mathrm{i}}$. The latter definition is preferred because it can be obtained from the two other fixation indices, F_ST and F_IT and because it is unaffected by the mean gene frequency. The expected frequency of heterozygotes in small subpopulations of dioecious organisms will exceed Hardy-Weinberg expectations and this can be measured by $\text{F}\text{̂}{}_{\mathrm{IS}}$. In an isolated subpopulation of constant variance effective size $\text{N,}\text{F}\text{̂}{}_{\mathrm{IS}}$ rapidly tends to $\text{1 − 4N}{}^2\text{(N − 1 + [N}{}^2\text{+ 1]}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{)}{}^2$. In the Island model of population structure, $\text{F}\text{̂}{}_{\mathrm{IS}}$ is approximately $\text{−(1 − m)}\text{N}\text{where}\text{m}$ is the immigration rate.When a sample is drawn from a natural population, the observed F_IS will depend upon the genetic structure of the population. The values of F_IS expected in three different types of population structure are discussed.     

Reactivity of chemically generated superoxide radical anion with peroxides as determined by competition kinetics

A steady-state competition system has been developed to investigate the reactions of the superoxide radical anion ($\text{O}{}_2{}^{\mathrm{<mtext>?</mtext>}}$) with various peroxides, including the so-called Haber-Weiss reaction. Potassium superoxide dissolved in an oxygen-free solution of DMSO containing 18-dicyclohexyl-6-crown, is the source of $\text{O}{}_2{}^{\mathrm{<mtext>?</mtext>}}$. High pressure liquid chromatography is used as an assay system for $\text{O}{}_2{}^{\mathrm{<mtext>?</mtext>}}$ reactivity, to detect and quantitate the yield of anthracene, formed as a major product in the reaction between $\text{O}{}_2{}^{\mathrm{<mtext>?</mtext>}}$ and 9,10-dihydroanthrancene. Decrease in anthracene yields, in the presence of peroxide, may be used to indicate a possible competing reaction between $\text{O}{}_2{}^{\mathrm{<mtext>?</mtext>}}$ and added peroxide. Complications involving peroxide-stimulated formation of anthraquinone derivatives are discussed. No evidence for a competing reaction between $\text{O}{}_2{}^{\mathrm{<mtext>?</mtext>}}$ and peroxide can be detected up to a 10-fold excess of peroxide over 9,10-dihydroanthracene.     

Protein-catalyzed phospholipid exchange in bilayer vesicles determined by flow cytometry and electron microscopy

The protein-induced lipid transfer between phosphatidylcholine vesicles was investigated. Measurements of the degree of polarization at single vesicles were made by flow cytometry using diphenylhexatriene as the optical probe. Vesicles differing in phase transition temperature could be distinguished by their degree of polarization at a temperature where one population was in the fluid ($\text{T > T}{}_{\mathrm{<mtext>t</mtext>}}$) and the other one in the quasi-crystalline ($\text{T < T}{}_{\mathrm{<mtext>t</mtext>}}$) state. Besides vesicles containing exchanged lipids we also observed fractions of unaffected vesicles. The lipid exchange was visualized directly by freeze-fracture electron microscopy. The characteristic ‘ripple’ structure of phosphatidylcholine vesicles disappeared upon exchange with lipid in the fluid state.     

Synthesis of a complementary DNA to rat liver albumin mRNA.

NADPH reduces both liver microsomal cytochrome P-450 and cytochrome $\text{b}{}_5$. In the presence of CO, ferrous cytochrome P-450 can slowly transfer electrons to amaranth, an azo dye. This reaction is followed by the reoxidation of cytochrome $\text{b}{}_5$ which proceeds at essentially the same rate as does cytochrome P-450 oxidation. It is suggested that cytochrome $\text{b}{}_5$ directly reduces cytochrome P-450 in rat liver microsomes.     

The effect of formate on cytochrome aa3 and on electron transport in the intact respiratory chain

1. Formate inhibits cytochrome $\text{c}$ oxidase activity both in intact mitochondria and submitochondrial particles, and in isolated cytochrome $\text{aa}{}_3$. The inhibition increases with decreasing pH, indicating that HCOOH may be the inhibitory species.2. Formate induces a blue shift in the absorption spectrum of oxidized cytochrome $\text{aa}{}_3\text{(a}{}^{\mathrm{3+}}\text{a}{}_3{}^{\mathrm{3+}}$) and in the half-reduced species ($\text{a}{}^{\mathrm{2+}}\text{a}{}_3{}^{\mathrm{3+}}$). Comparison with cyanide-induced spectral shifts, towards the red, indicates that formate and cyanide have opposite effects on the $\text{aa}{}_3$ spectrum, both in the fully oxidized and the half-reduced states. The formate spectra provide a new method of obtaining the difference spectrum of $\text{a}{}_3{}^{\mathrm{2+}}$ minus $\text{a}{}_3{}^{\mathrm{3+}}$, free of the difficulties with cyanide (which induces marked high → low spin spectral shifts in cytochrome $\text{a}{}_3{}^{\mathrm{3+}}$) and azide (which induces peak shifts of cytochrome $\text{a}{}^{\mathrm{2+}}$ towards the blue in both α- and Soret regions).3. The rate of formate dissociation from cytochrome $\text{a}{}^{\mathrm{2+}}\text{a}{}_3{}^{\mathrm{3+}}\text{-}\text{HCOOH}$ is faster than its rate of dissociation from $\text{a}{}^{\mathrm{3+}}\text{a}{}_3{}^{\mathrm{3+}}\text{-}\text{HCOOH}$, especially in the presence of cytochrome $\text{c}$. The $\text{K}{}_{\mathrm{<mtext>i</mtext>}}$ for formate inhibition of respiration is a function of the reduction state of the system, varying from 30 mM (100% reduction) to 1 mM (100% oxidation) at pH 7.4, 30 °C.4. Succinate-cytochrome $\text{c}$ reductase activity is also inhibited by formate, in a reaction competitive with succinate and dependent on [formate]².5. Formate inhibition of ascorbate plus $\text{N,N,N′,N′-}\text{tetramethyl}\text{-p-}\text{phenyl-enediamine}$ oxidation by intact rat liver mitochondria is partially released by uncoupler addition. Formate is permeable through the inner mitochondrial membrane and no differences in ‘on’ or ‘off’ inhibition rates were observed when intact mitochondria were compared with submitochondrial particles.6. NADH-cytochrome $\text{c}$ reductase activity is unaffected by formate in submitochondrial particles, but mitochondrial oxidation of glutamate plus malate is subject both to terminal inhibition at the cytochrome $\text{aa}{}_3$ level and to a slow extra inhibition by formate following uncoupler addition, indicating a third site of formate action in the intact mitochondrion.