I. Purification and properties of supernatant and mitochondrial malate dehydrogenase from the brown shrimp, Penaeus aztecus. II. Ultracentrifugation studies of proteins using a photoelectric scanning system

Part I The supernatant and mitochondrial forms of muscle malate dehydrogenase have been isolated from the brown shrimp, Penaeus aztecus. Both the supernatant and mitochondrial enzyme fractions were homogeneous in ultracentrifugal analysis. The s[subscript 20, w] values for the supernatant and mitochondrial enzymes are 4.9 S and 4.7 S; while their molecular weights are 75,800 and 69,300. The electrophoretic and urea denaturation properties of these enzymes suggest that they are dimeric in structure. The two enzymes are similar in a number of catalytic properties. Both enzymes are partially inhibited by high oxaloacetate concentration and are greatly inhibited by high L-malate concentration. The supernatant enzyme is relatively thermostable at 45[degrees]C but the mitochondrial enzyme is heat labile. The apparent K[subscript m] values for oxaloacetate reduction and L-malate oxidation have been determined. In many respects, the shrimp enzymes have catalytic properties similar to those found for the corresponding malate dehydrogenases of various vertebrate species. Part II It is possible to obtain corrected sedimentation coefficients (s[subscript 20, w]) with protein concentrations of 0.075 mg/ml by using the photoelectric scanning system of the Beckman model E analytical ultracentrifuge. The values obtained are comparable with the literature data. No effect due to thermal convection in the sample cell is observed in the protein boundary sedimentation velocity experiments. In low speed equilibrium sedimentation of dilute protein solutions, the effects of thermal convection in the sample cell are overcome by using a modified temperature system which heats the rotor uniformly from all directions. Analysis of the photoelectric scanner tracings and calculations of protein molecular weights is much easier but somewhat less accurate than analysis of similar data from the Rayleigh optical system. A reported decrease in corrected protein sedimentation coefficients, in experiments conducted below room temperature, was confirmed. It has been reported that this effect is due to the design of the temperature control system ofthe Beckman instrument. However, this study provides evidence that this effect is attributable to a conformational change in the sedimenting macromolecular species.