Influence of exogenous substrates on the endogenous respiration of Pseudomonas aeruginosa

Gronlund, Audrey F. (University of British Columbia, Vancouver, Canada), and J. J. R. Campbell. Influence of exogenous substrates on the endogenous respiration of Pseudomonas aeruginosa. J. Bacteriol. 91:1577-1581. 1966.-The influence of growth conditions, ammonium ions, and glucose concentration on endogenous respiration in Pseudomonas aeruginosa was determined by measuring C(14)O(2) evolution from uniformly labeled cells that had previously been grown on C(14)-glucose. A 93% suppression of endogenous C(14)O(2) evolution was evident under growth conditions, and a 66% suppression was observed in the presence of excess glucose. Increasing exogenous glucose concentrations supported decreasing levels of endogenous C(14)O(2) evolution. Ammonium ions slightly suppressed endogenous activity and enhanced the decrease in C(14)O(2) release observed with exogenous glucose. In addition, the effect of exogenous glucose, alpha-ketoglutarate, 2-ketogluconate, aspartic acid, and adenosine selectively on both endogenous ribonucleic acid (RNA) and protein oxidation was followed by measuring C(14)O(2) evolution from cells grown with C(14)-uracil or C(14)-proline. The five exogenous substrates examined suppressed endogenous RNA oxidation, and the degree of suppression appeared to be correlated with the amount of oxygen consumption and, hence, energy gained during the oxidation of these substrates. Oxidation of endogenous protein was decreased when cells were incubated with glucose, aspartate, and adenosine, but was increased when alpha-ketoglutarate and 2-ketogluconate were the exogenous substrates. The influence of the oxidizable exogenous compounds appeared to be related, in part, to the ammonium ion requirement imposed upon the cells for assimilation of the individual exogenous substrate.     

Cyanide-resistant respiration in Torulopsis candida

The effect of cyanide and the inhibitors of cyanide-resistant oxidase--hydroxamic acids on endogenous respiration and oxidation of a number of substrates by Torulopsis candida resting cells taken at different stages of growth on glucose and hexadecane was studied and made it possible to arrive at the following conclusions. 1. The effect of cyanide on endogenous respiration of T. candida differs during its growth on glucose and hexadecane. On hexadecane, irrespective of the growth phage, cyanide inhibits endogenous respiration by 70--75%. On glucose, cyanide inhibits endogenous respiration not more than by 35% and only at the exponential growth phase whereas it stimulates endogenous respiration in the course of other growth stages. 2. The effect of cyanide on respiration of the resting cells of T. candida which oxidize glucose, hexadecane, primary alcohols and tetradecanoic acid hardly depends on the growth stage. It is determined mainly by the nature of a substrate to be oxidized. 3. Hydroxamic acid have no effect on the cell respiration in the absence of cyanide. However, in its presence, they entirely inhibit both endogenous respiration and oxidation of the aforementioned substrates. 4. Under almost all above experimental conditions, the sensitivity of cell respiration to cyanide changes only slightly at different stages of growth on either glucose or hexadecane. This feature markedly distinguishes T. candida among other cyanide-resistant yeasts.     

Respiration in cell development

Summary Dark oxygen uptake was measured manometrically for cells of green high-temperature alga, Chlorella 7-11-05, separated from nonsynchronized populations by centrifugation into fractions of predominantly small or large cells. In the presence of exogenous glucose, respiration activity of the smaller (younger) cell fraction was invariably higher than that of the larger (older) cell fraction. In the absence of exogenous substrate, the difference in respiration rates in two fractions of cells was inconsistent from one experiment to another both in size and in sign. The dependence of dark respiration on the amount of available substrate makes the endogenous respiration rate unsuitable as an indicator of the inherent capacity of respiratory mechanisms.In observations on synchronized heterotrophically grown cells, the glucose respiration rate expressed per dry weight of cells gradually declined over the developmental period irrespective of the adequate exogenous supply of glucose or illumination by weak light. Observations on synchronized heterotrophically grown Chlorella cells thus corroborated studies of glucose respiration in cells separated into are groups by centrifugation.The decline in metabolic activity in the course of cell development previously established for growth and photosynthesis extends to include respiration activity. Disagreements among several investigators in regard to the course of respiration during cell development are probably due to the effects of accessory factors such as strong light during the preceding growth period or the scarcity of respiratory substrate during respiration measurements which affect and distort changes in the inherent capacity of metabolic mechanisms in the course of cell development.     

Glucose metabolism of thraustochytrium roseum,a nonfilamentous marine phycomycete

Summary In uniformly labeled logarithmic-phase cells of Thraustochytrium roseum grown in isotopic glucose, 85% of the respiratory CO₂ was derived from endogenous reserves and only 15% was contributed by exogenous glucose. Experiments with asymetrically labeled glucose showed that the main portion of metabolic CO₂ came from carbon 1 of the glucose molecule, suggesting that the hexose monophosphate shunt is a major pathway for glucose dissimilation in the fungus. The presence of several enzymes of the hexose monophosphate shunt, the Embden-Meyerhof and glyoxylate pathways, and the tricarboxylic acid cycle were demonstrated.     

The metabolic properties of spermatozoa from the epididymis of the tammar wallaby,Macropus eugenii

The utilization of various substrates by sperm from the cauda epididymidis of the tammar was examined because the major naturally occurring sugar in the semen of this species is N-acetyl-D-glucosamine (NAG) and not fructose, as in eutherian mammals. The sperm displayed a high level of endogenous respiration that supported motility for relatively prolonged periods of time in vitro. They also metabolised exogenous ¹⁴C-labelled glucose, NAG, sucrose, and acetate through glycolytic and/or oxidative processes to produce lactate and ¹⁴CO₂ at varying rates. The rate of uptake of NAG by tammar sperm was about four times greater than that of other substrates. Glucose and/or NAG stimulated the rate of oxygen consumption by about 20%, but acetate stimulated oxygen consumption by more than 40%. The most striking findings were that NAG almost completely inhibited the oxidation of glucose and sucrose by the sperm and depressed the uptake of glucose, 3-O-methylglucose, and sucrose. Acetate oxidation also was inhibited by NAG, but only by about 50%. Tammar sperm generated substantial amounts of free glucose during incubation with NAG, but this and the inhibitory effects of NAG on glucose oxidation were not mimicked by rat sperm. It is proposed that tammar sperm fail to oxidise glucose in the presence of NAG because of the rapid cellular uptake of NAG relative to glucose. Also, the intracellular glucose and acetate liberated from NAG would compete with exogenous glucose for processing in the Embden-Meyerhof and tricarboxylic acid (TCA) cycle pathways. It is also suggested that tammar sperm oxidise sucrose after extracellular hydrolysis into its glucose and fructose components. The biological implications of these metabolic and transport properties of tammar sperm have as yet to be determined. Mol. Reprod. Dev. 49:92–99, 1998. © 1998 Wiley-Liss, Inc.     

The role of sugars in the respiration of differently cultivated green algae

A study of the effect of exogenous sugars (fructose, glucose, arabinose and xylose) on Qo₂ inChioreila pyrenoidosa (A 82) and inScenedesmus obliquus (A 125) showed that the effect of sugars on respiration depends on previous cultivation of the alga. In autotrophically cultivated algae all sugars contribute to the increase of Qo₂, in those cultivated rnixotrophically (with the addition of glucose or together with yeast extract, and with xylose) this occurs only in Scenedesmus when glucose or xylose are applied. The effect of cultivation was apparent in that mixotrophically cultivated algae had larger endogenous reserves of substrates than the autotrophic ones. The exogenous substrate is not preferentially respired and does not affect respiration solely as its substrate. In Chlorella, the effect of different cultivation conditions is more pronounced, the QO₂ being usually higher than inScenedesmes.     

Studies on the Metabolism of Aspergilli

Studies were made on the endogenous respiration of Aspergillus sojae K.S. Observing the changes of Kjeldahl-nitrogen in each fraction of the mycelial components, the author concluded that pool amino acids, bound amino acids, protein, nucleic acids and nucleotides covered whole of the nitrogenous reserves available for endogenous respiration in the mycelia. A study was carried out on the effect of preincubation with glucose or amino acids on endogenous respiration. Stimulation of either oxygen uptake, protein breakdown or ammonia formation was observed during respiration of the mycelia incubated with a suitable concentration of azide, 2,4-dinitrophenol, potossium fluoride, monoiodoacetic acid or ethylenediaminetetraacetic acid. Ammonia formation accompanied with endogenous respiration seemed to proceed inversely by the influence of energy yielding reaction.     

The tricarboxylic acid cycle and glycolysis in relation to ion transport by the ciliary body

1. The respiration and aerobic glycolysis of pig ciliary processes in oxygenated phosphate and bicarbonate buffers have been investigated. 2. Significant amounts of lactic acid are produced only in the presence of added glucose, but this does not change the endogenous respiration rate. 3. Succinate and citrate increase the oxygen uptake considerably, but pyruvate has almost no effect; oxaloacetate and fumarate stimulate slightly in the presence of glucose. Aspartate and fumarate together stimulate pyruvate utilization and are oxidized as fast as citrate. 4. Ouabain inhibits the oxidation of glucose and other substrates by limiting the ADP supply from the sodium transport system. Cyanide and azide inhibit respiration and stimulate glycolysis. 5. The transport mechanism depends largely on ATP from oxidative phosphorylation and regulates the rate of respiration and glycolysis by controlling ADP production from the Na(+)-K(+)-activated adenosine triphosphatase.     

The effect of carbonyl cyanidep-trifluoromethoxy-phenylhydrazone on yeast cells

Carbonyl cyanidep-trifluoromethoxy-phenylhydrazone (FCCP) induced anaerobic fermentation of endogenous reserves and stimulated endogenous respiration in yeast. Under the given experimental conditions it stimulated the oxidation of glucose in concentrations of 0.4–3 μM; higher concentrations inhibited the oxidation and fermentation of glucose. The oxidation of pyruvate, ethanol andd(?) and L(+)-lactate was already inhibited by very low FCCP concentrations. Phosphorylation coupled to oxidation of succinate in subcellular particles was uncoupled by 0.1–1 μM FCCP. The effect on the intact cell depended on the ratio of the FCCP and cell concentration, since FCCP was absorbed from the medium by the cells. Absorption of FCCP by the cells was completely prevented by cysteine, which also blocked the inhibitory effect of FCCP on the metabolism of whole cells.     

SPORE GERMINATION AND CARBON METABOLISM IN FUSARIUM SOLANI. II. ENDOGENOUS RESPIRATION IN RELATION TO GERMINATION

Cochrane , V. W., Jean C. Cochrane , C. b . Collins , and F. G. Serafin . (Wesleyan U., Middletown, Conn.) Spore germination and carbon metabolism in Fusarium solani. II. Endogenous respiration in relation to germination. Amer. Jour. Bot. 50(8): 806–814. Illus. 1963.—Endogenous oxygen uptake by ungerminated macroconidia of Fusarium solani f. phaseoli is more than doubled by exogenous ammonium ion and is increased about 7-fold by germination. The respiratory quotient is halved by the provision of ammonia, which has essentially no effect on the endogenous formation of carbon dioxide. Stimulation by azide and 2,4-dinitrophenol suggests that the supply of phosphate acceptors regulates the rate of endogenous respiration. Mercurials poison the endogenous respiration, cyanide accelerates it, and iodoacetate, arsenite, fluoride, and chelating agents have little effect. Respiration is little affected by changes in pH, external phosphate, oxygen concentration, and spore density, within the limits tested. Spore lipid concentration is increased by cultivation in a high-glucose medium, but this variation in lipid content of spores docs not affect the endogenous Qo₂, nor does a high lipid content abolish the requirement for exogenous carbon for germination. Lipid utilization during endogenous respiration accounts for 37% of the loss in dry weight; lipid is also utilized during germination, but such utilization amounts to only about 5% of the carbon requirement. Neither mannitol nor nitrogenous compounds are significant substrates of endogenous respiration. The oxidation of the exogenous substrates tested does not measurably affect the concomitant rate of endogenous respiration. It is proposed that endogenous respiration can contribute to the synthetic processes of spore germination, although quantitatively insufficient to support germination without exogenous carbon. It is also questioned whether the respiratory quotient is an adequate index of the substrate of endogenous respiration.     

Comparative Physiology of Respiration in Aquatic Fungi II. The Saprolegniales, Especially Aphanomyces astaci

The endogenous respiration of six members of Saprolegniaceae (Oomycetes), Saprotegnia sp., Thraustotheca sp., Achlya sp., Dichtyuchus sp., Aphanomyces euteiches, and A. astaci were studied in the presence and in the absence of exogenous substrate using conventional manometric techniques. Glucose stimulated the rate of oxygen uptake of unstarved mycelia to some extent in all the fungi. Attempts to increase the weak stimulation of respiration by glucose in A. astaci were not successful. The respiratory quotients of the fungi tested were usually in the range of 0.7 to 0.9 during endogenous respiration, and addition of glucose increased these values more than expected. L-leucine and L-glutamic acid stimulated respiration of A. astaci only when the fugus was starved, and acetic acid and butyric acid were inhibitory. Fructose and acetic acid increased respiration in starved mycelium of A. euteiches while L-leucine and L-glutamic acid had little effect. Antimycin A, HOQNO, HCN, and fluoroacetate strongly inhibited endogenous oxygen uptake by A. astaci. Amytal and azide were also markedly inhibitory while rotenone and CO had little effect. DNP and diphenylamine inhibited respiration at a high concentration but at a lower concentration DNP was stimulatory. In contrast the respiration of Saprolegnia sp. was resistant to cyanide, antimycin A, and HOQNO. Spectrophotometric observations on homogenized mycelia of Saprolegnia sp. and of A. astaci indicated the presence of cytochrome c (551 nm), two b-type cytochromes (557 and 564 nm) and cytochrome a-a₃ (605 nm) all in approximately equimolar concentrations. In both strains CO combined with cytochrome a₃ and an unidentified pigment. A remarkable similarity in the cytochrome system seems to exist between these two strains and some members of the Leptomitales.     

Synaptosomal bioenergetics. The role of glycolysis, pyruvate oxidation and responses to hypoglycaemia

The bioenergetic interaction between glycolysis and oxidative phosphorylation in isolated nerve terminals (synaptosomes) from guinea-pig cerebral cortex is characterized. Essentially all synaptosomes contain functioning mitochondria. There is a tight coupling between glycolytic rate and respiration: uncoupler causes a tenfold increase in glycolysis and a sixfold increase in respiration. Synaptosomes contain little endogenous glycolytic substrate and glycolysis is dependent on external glucose. In glucose-free media, or following addition of iodoacetate, synaptosomes continue to respire and to maintain high ATP/ADP ratios. In contrast to glucose, the endogenous substrate can neither maintain high respiration in the presence of uncoupler nor generate ATP in the presence of cyanide. Pyruvate, but not succinate, is an excellent substrate for intact synaptosomes. The in-situ mitochondrial membrane potential (delta psi m) is highly dependent upon the availability of glycolytic or exogenous pyruvate; glucose deprivation causes a 20-mV depolarization, while added pyruvate causes a 6-mV hyperpolarization even in the presence of glucose. Inhibition of pyruvate dehydrogenase by arsenite or pyruvate transport by alpha-cyano-4-hydroxycinnamate has little effect on ATP/ADP ratios; however respiratory capacity is severely restricted. It is concluded that synaptosomes are valuable models for studying the control of mitochondrial substrate supply in situ.     

The effects of colloids on the appearance and substrate permeability of rat liver mitochondria

The electron microscopic appearance of rat liver mitochondria fixed in glutaraldehyde is altered if certain colloids (serum albumin, dextran or Ficoll) are present in the medium at about 3%. To compare behaviour in control and albumin-supplemented media, the rate of stimulated respiration was measured with various substrates. It was found that the rate of respiration was reduced with succinate or pyruvate and was almost abolished with oxoglutarate, while malate oxidation (in presence of glutamate) was unaffected. The rate of oxoglutarate oxidation could be restored by causing mitochondrial swelling. It is suggested that the effects are due to the presence of endogenous colloids in the particles whose effects on water activity have to be balanced by external colloid. In the absence of external colloid, swelling of the internal colloid-containing compartments may give rise to an enhanced permeability of the membrane so that reactions occurin vitro which do not take place rapidly if at allin vivo.     

On the catabolic pathways of sugars in green algae

Respiratory metabolism of the cultures of algaeChlorella pyrenoidosa (82) andScenedesmus obliquus (125) was investigated. One part of algae were cultivated on a mineral nutrient solution, another two parts on the solution with glucose and on the solution with glucose and yeast decoction. Individual steps of respiratory metabolism—endogenous as well as in the presence of exogenous sugars—were estimated according to the response to the following inhibitors: monoiodacetate, NaF, NaN₃, and 2,4-DNP. In the last two cases, fructose, glucose-6-phosphate and fructose-1,6-diphosphate were applied in parallel for comparison. Na-monoiodacetate was found to inhibit the respiration of both endogenous and exogenous (with glucose) substrates, NaF (concentrations up to 2.5.10^?2 m) stimulated the O₂ uptake. The effect of sodium azide and 2,4-DNP depended in both strains on previous cultivation. On the basis of the results obtained, the presence of particular respiration pathways in the dissimilation of glucose is discussed. The following catabolic processes are to be considered: a) direct oxidation (with both autotrophically cultivated strains and with theChlorella cultivated on glucosecontaining medium), b) the process similar to glycolysis, which, however, does not necessarily involve the enolase (it is not inhibited by NaF) c) pentosephosphate cycle (Chlorella), and d) glycolysis, in which both algae can operate when sugars previously phosphorylated are applied.     

Studies on the endogenous metabolism and senescence of starved Sarcina lutea

1. When washed suspensions of Sarcina lutea are starved aerobically in phosphate buffer at the growth temperature of 37 degrees , the rate of endogenous oxygen consumption decreases to very low values after 10hr., although many of the cells survive for 40hr. If starvation is prolonged further, the bacteria die at a rate of approximately 1.5% of the initial viable population per hour. 2. Oxidation of intracellular free amino acids accounts for most of the observed endogenous oxygen uptake but RNA is also utilized and a portion of the component bases and pentose is degraded and presumably oxidized. Ammonia appears in the supernatant and some pentose and ultraviolet-absorbing nucleotide are released from the cells. DNA, protein and polysaccharide are not measurably degraded. 3. Survival can be correlated with the ability of aerobically starved bacteria to oxidize exogenous l-glutamate and glucose. When starved under nitrogen for 40hr. cells continue to oxidize their endogenous reserves at undiminished rates when transferred to aerobic conditions; on prolonging anaerobic starvation the rate of oxidation declines during the period of most rapid loss of viability. 4. In the presence of Mg(2+), RNA degradation during aerobic starvation is almost completely suppressed without affecting the period for which the bacteria survive. 5. Cells grown in peptone supplemented with glucose accumulate reserves of polysaccharide which are metabolized in aerobic starvation, together with free amino acids. Ammonia is evolved and RNA is degraded to a greater extent than in peptone-grown suspensions. Bacteria rich in polysaccharide survive less well than those which are deficient in the polymer; the reason for this phenomenon has yet to be established. 6. In peptone medium, endogenous oxygen uptake and the concentration of intracellular free amino acids decline as growth progresses and they continue to decrease when the organism is held in stationary phase. Under the conditions used, the endogenous Q(o2) and free amino acid pool of cells grown in peptone with 2% (w/v) glucose did not decline so markedly and the bacteria contained large amounts of polysaccharide at all stages of growth.     

Radiochemical investigation of the respiration of spores of Bacillus cereus

Summary The endogenous respiration of ¹⁴C-labelled spores of B. cereus was measured through the ¹⁴CO₂ produced, and the rate expressed as Q (l CO₂/hxmg). New upper limits for respiration in various conditions have been set.Dry spores had no measurable activity; Q<10^–4 at room temperature and <10^–3 at 35° C. For wet spores of different harvests, at 30°C, Q lay between 0.0013 to 0.067. Near 40° C, respiration showed a maximum. Thermal history has a great influence on Q. CO₂ production by heat-killed spores is attributed largely to infection.Water or 10^–3 m sodium phosphate buffer (pH=6.5) gave equal spore respiration, in strong NaCl it was less. Azide enhanced respiration dramatically. A temporary increase was also found with non-radioactive glucose. Exogenous respiration of spores in glucose exceeded endogenous respiration.Endogenous and exogenous respiration of vegetative forms were much larger than those of spores and were time-dependent. The ratio of minimum (endogenous, dry spores) and maximum (exogenous, wet vegetative cells) respiration was at least 3x10⁵.     

Role of glucose metabolism in acid formation by isolated gastric glands

The role played by glucose in providing energy for acid formation was studied in isolated gastric glands from rabbit. The widely-used inhibitors of glycolysis, iodoacetic acid and iodoacetamide were found to inhibit glucose oxidation as well as the indicators of acid formation, respiration and accumulation of aminopyrine. However, the potent inhibition of acid formation was found to involve a nonspecific mechanism other than the simple inhibition of glycolysis. An alternative approach involved use of the glucose transport inhibitor, phloretin. Phloretin blocked glucose oxidation and also inhibited functional responses. Acid formation was restored easily by the addition of pyruvate or various other oxidizable substrates. Measurement of lactate formation in the absence of exogenous glucose showed that the gastric glands contain very little glycogen. Addition of external glucose resulted in a 10-fold increase in lactate formation and this rate was stimulated further by histamine and rotenone. Rotenone also inhibited both respiration and aminopyrine accumulation; however, the inhibition was not complete. Phloretin treatment resulted in total inhibition of the residual aminopyrine accumulation after rotenone treatment. The results are interpreted to indicate that gastric glands are dependent almost totally on external substrate supply to support acid formation; and, that while anaerobic glucose metabolism can sustain a very low level of acid formation, the major role of glucose is to yield pyruvate equivalents for subsequent oxidation.     

Some Specific Features of the Respiration of Hydrocarbon-utilizable Candida Yeasts

The respiration of both glucose-grown and hydrocarbon-grown cells of Candida tropicalis pK 233 harvested in the stationary phases was not inhibited by cyanide when glucose was used as oxidation substrate, but the former was rather stimulated in the presence of cyanide. When n-alkanes were used as oxidation substrate, cyanide lowered the respiratory activities of both cells to about 50%. With respect to the susceptibility to cyanide, the younger cells growing on n-alkanes were less sensitive in hydrocarbon oxidizing ability than the older cells, whereas the older cells growing on glucose or n-alkanes were more resistant in glucose oxidizing ability than the younger cells. Acetate was oxidized by both glucose-grown and hydrocarbon-grown cells of the yeast. Laurate was oxidized by hydrocarbon-grown cells, but not by glucose-grown cells. The respiration on laurate was inhibited completely by 3.3 mM of cyanide. In general, hydrocarbon-grown cells of Candida tropicalis pK 233 were more sensitive to various respiratory inhibitors than glucose-grown cells, although the oxidation substrates had a significant effect.

The respiration of both glucose-grown and hydrocarbon-grown cells of C. albicans, C. guilliermondii and C. lipolytica harvested in the stationary phases was also resistant to cyanide when glucose was used as oxidation substrate. But the respiration on n-alkanes of these cells was inhibited significantly by 3.3 mM of cyanide except for C. albicans.     

Branched respiratory chain in aerobically grown Staphylococcus aureus —oxidation of ethanol by cells and protoplasts

Addition of ethanol and some other primary alcohols, except methanol, to cells and protoplasts (but not membrane particles) considerably stimulated the rate of oxygen consumption. This additional respiration was strongly inhibited by 0.1 mM KCN. The cyanide inhibition curve of endogenous substrate oxidation was slightly biphasic while in the presence of ethanol it became clearly biphasic having K _i values of approx. 0.1 and 0.5 mM. Based on the steady-state cytochrome spectra in the presence of 0.1 mM KCN, we attributed the lower K _i to cytochrome a ₆₀₂. Proteolysis of protoplasts external membrane proteins did not change the rate of endogeneous substrate oxidation but prevented the inhibition of this respiration by low concentrations of KCN and stimulation of oxygen consumption by ethanol. The activity of NAD⁺-dependent ethanol dehydrogenase in the cytoplasm was found to be 520 nmol NADH-x min^–1 x mg^–1 protein. Proteolysis of external membrane proteins apparently inhibits the operation of the cytochrome a ₆₀₂-containing electron transport branch inducing the suppression of electron flow from NADH to oxygen.     

The Effect of Cadmium on Electron and Energy Transfer Reactions in Corn Mitochondria

The effects of cadmium on isolated corn shoot mitochondria were determined. In the absence of phosphate cadmium stimulated the oxidation of exogenous NADH optimally at 0.025 mM, but was inhibitory at 0.1 mM and above. The presence of phosphate negated the cadmium stimulation of exogenous NADH oxidation and permitted inhibitions only at higher cadmium concentrations. Succinate or malate + pyruvate oxidation in the absence of phosphate was inhibited to a greater extent by cadmium than when phosphate was present. ADP/O and respiratory control ratios were reduced by cadmium but generally were less sensitive to cadmium than state 4 or minus phosphate respiration. The data suggest that the site of cadmium effect is likely to be early in electron transport. Cadmium had a pronounced effect on mitochondrial swelling under either passive or active conditions. When succinate or exogenous NADH were being oxidized swelling occurred at 0.05 mM cadmium, but with malate + pyruvate the cadmium concentration had to exceed 1.0 mM. Phosphate (2 mM) prevented the swelling. Dithiothreitol, a SH group protector, prevented any effect of cadmium on swelling or respiration which suggests that sulfhydryl groups are likely involved in the cadmium-membrane interaction.