The large number of inhibitors of cellular respiration hitherto described have been reported to act by inhibiting the activity of enzyme systems. They do so (1) by combining with the activating protein, either through some groups essential for activity (for example, the -S H groups), through denaturation of the molecule, or by combination on the side chains where substrates or prosthetic groups form the protein-substrate complex (structural inhibitors); (2) by combining with the prosthetic groups of enzymes (diphosphothiamine, pyridoxal, pantothenic acid, etc.); (3) by combining with the series of oxidation reduction systems (pyridine nucleotides, flavins, cytochromes) which transfer electrons from oxidizable substrate to molecular oxygen. Besides this direct action on the components of enzyme systems, cellular respiration may be affected by alteration of the varied mechanisms which regulate in the living cells the rate and the direction of enzymatic reactions. One of these regulating mechanisms is the state of the cellular membrane. It is generally agreed that the cell membrane is a lipoid-protein system possessing varying degrees of permeability where penetration occurs by passage through the pores of the membrane or through solution in the lipoid portion. Any alteration of the solubility coefficient in the lipoid phase or changes of the pore size will bring forth alterations in the rate of passage of substrates, and as a consequence alterations in the metabolsim of the cell. We present in this paper experiments on the inhibition of cellular oxidations produced by uranyl nitrate, which have been interpreted as being due to combination of uranium with the protein layer of the cell membrane, bringing thus an increased impermeability to the passage of certain oxidizable substrates. EXPERIMENTALThe yeast cells used in these experiments were brewers' yeast from Keely Brewing Company, Chicago, and bakers' yeast from Fleischmann. The first fermented

It has been known for some time (Risse, 1929;Fricke, 1934) that when water is irradiated with x-rays in the presence of oxygen there is formation of H 2 O2. Since H 2 O2 is a powerful oxidizing agent and it easily oxidizes sulfhydryl groups, it was reasonable to assume that this substance, if formed on irradiation, would contribute to the biological effects of ionizing radiations. In fact, Barren and Dickman (1949) on studying enzyme inhibitions by ionizing radiations, and Barron and Flood x on studying the oxidation of 2,3-dithiopropane and of glutathione by x-rays, were able to distinguish the H 2 O 2 contribution to this oxidation by the use of catalase. The French investigators Caillot (1941), andLatarjet (1942) have postulated that the primary effect of irradiation in aqueous solutions is H 2 O 2 formation, which would thus become of importance in the interpretation of the mechanism of ionizing radiations. The same view is held by Evans (1947) who found that the fertilizing power of sea urchin sperm is decreased when suspended in sea water irradiated with large doses of x-rays. Evans attributed this inhibition to the action of H 2 O 2 seemingly formed on irradiation of sea water. In living .cells the role of H 2 O 2 becomes more complicated because the sulfhydryl groups which might be oxidized by this agent not only are present in the protein moiety of enzymes, but also exist as non-protein sulfhydryl groups. We present in this paper experiments on the effect -of H 2 O 2 and of x-ray irradiated sea water on the respiration of sea urchin sperm. They do not support the belief that H 2 O 2 is an important factor in x-ray toxicity on sea urchin sperm. EXPERIMENTALSea urchin sperm was obtained as described previously , and in all experiments a dilution of 1 : 200 was used. Freshly filtered sea water was irradiated at room temperature in large cellophane dishes and immediately after irradiation the sperm suspension was added, enough to make the desired dilution of 1 : 200.The catalase added to irradiated sea water was prepared from beef liver according to Sunmer and Dounce (1939). H 2 O 2 was determined by the colorimetric method of Bonet-Maury (1944). An aliquot of the solution was taken (up to 1 Unpublished experiments.