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1. In addition to the known 2R,3R- and 2R, 3S-2,3-dihydroxy-3-methylpentanoic acids (DHI), the 1S,3S- and sS,DR-isomers were prepared. 2S-2,3-Dihydroxy-3-methylbutanoic acid (DHV) was also prepared in addition to the known 2R-isomer. 2. The six dihydroxy acids were examined for their ability to promote the growth of isoleucine-valine (ilv)-requiring strains of Salmonella typhimurium and to serve as substrates for the alpha,beta-dihydroxyacid dehydratase of the same organism. 3. Only 2R,3R-2,3-dihydroxy-3-methylpentanoic and 2R-2,3-dihydroxy-3-methylbutanoic acids supported growth of the ilv strains of S. typhimurium. 4. alpha,beta-Dihydroxyacid dehydratase utilized the three isomers with the 2R-configuration as substrates but not those with the 2S-configuration. 5. In an additional growth study that utilized the 3R- and 3S-isomers of 3-methyl-2-oxopentanoic acid, the alpha-keto acid analogue of isoleucine, only the 3S-isomer supported growth. 6. It is concluded that the mechanism of action of the dehydratase is stereospecific in that the proton that is attached to C-3 of the substrate occupies the same steriochemical position as the departing hydroxyl group (Fig. 6).

ATP hydrolysis, the exchange of inorganic phosphate with ATP, and ATP synthesis have been studied as a function of Mg2+ concentration in bovine heart submitochondrial particles. The rate of exchange is low at concentrations of Mg2+ below 3 mM, at higher concentrations, the exchange is several times higher. ATP hydrolysis shows a different pattern with respect to the concentration of Mg2+. The ratio of ATP hydrolyzed to ATP exchanged is above 20 at Mg2+ concentrations below 3 mM and about 5 at high Mg2+ concentrations; ADP induces a further drop of the ratio (2-3). By assays of the sensitivity of the hydrolytic reaction to organic solvents (dimethyl sulfoxide), it has been possible to determine the rate-limiting step of ATP hydrolysis. At 3 mM Mg2+, the rate-limiting step is the release of ADP in the soluble, but not in the particulate enzyme. However at higher Mg2+ concentrations, the rate-limiting step in the particulate enzyme is also ADP release. Therefore, the decrease in the ratio of ATP hydrolysis to inorganic phosphate incorporated into ATP coincides with a change in the kinetics of the enzyme, i.e. when the terminal step of ATP hydrolysis becomes rate-limiting, the inorganic phosphate-ATP exchange increases. Ca2+ induces an increase in the phosphate-ATP exchange at low Mg2+ concentrations.