pH-dependent cold lability of rabbit skeletal muscle AMP deaminase

Role of hydrodynamic shear on activity and structure of proteins

Proteins are important products used in industry. They may be enzymes which catalyze different reactions or they may be required for their biological activities as hormones, growth factors or therapeutics. During production and recovery, proteins are subjected to fluid forces which arise due to operations such as stirring, pumping and centrifugation. The resulting hydrodynamic shear forces may cause damage to the large molecular weight proteins, resulting in denaturation and inactivation of the protein. This is a major concern as it affects the overall efficiency of protein recovery and final yield of the product. A considerable amount of research has been devoted to studying the effects of hydrodynamic shear stress on proteins, especially with respect to the enzymes. Enzymes are subjected to shear stresses during their production in fermentors, during isolation and purification steps in downstream operations and also during their use in enzyme reactors, especially if stirred reactors are employed to perform enzyme catalysed reactions. The present review discusses the effects of fluid shear stress on proteins including enzymes. A brief description on deactivation has been included in order to understand the effect of shear on the deactivation kinetics of proteins. The model systems used to subject proteins to shear and some unit operations during protein processing or use wherein they are exposed to shear stresses have also been presented. The significance of shear effects in designing bioprocesses involving shear sensitive biocatalysts as well as suggestions for future work have also been given.

Alteration of enzymes in ageing human fibroblasts in culture. V. Mechanisms of glutathione peroxidase modification

Ageing of WI-38 fibroblasts in culture was used as a model in order to investigate the evolution and the alteration of the key antioxidant enzyme glutathione peroxidase. The activity of glutathione peroxidase is influenced by the presence of selenium in the culture medium and we have also shown that the specific activity of this enzyme does not decrease during ageing, but rather slightly increases. No alteration could be detected by immunotitration. Also the kinetic parameter Km for tert-butyl hydroperoxide has not changed. However, the heat resistance of the enzyme dramatically decreases with ageing. Dilutions of the enzyme preparations had the same influence on the thermosensitivity of the enzyme. This dilution effect is most probably linked to the dissociation of the enzyme subunits into dimers and monomers. Moreover, the kinetic of thermoinactivation curves are best explained by consecutive reactions of inactivation with an intermediary enzyme form. These observations strongly support the hypothesis that ageing is associated with an increased dissociation constant of the tetrameric glutathione peroxidase leading to an easier dissociation of the enzyme in old cells.

AMP deaminase from equine muscle: purification and determination of regulatory properties

1. AMP deaminase from thoroughbred horse muscle was purified to apparent homogeneity and its regulatory properties were determined at pH 6.5 and 7.4. 2. The results are discussed in relation to the potential role of muscle AMP deaminase during exercise and the existence of two molecular forms depending on the pH.

Regulation of skeletal-muscle AMP deaminase. Evidence for a highly pH-dependent inhibition by ATP of the homogeneous derivative of the rabbit enzyme yielded by limited proteolysis

Limited proteolysis of rabbit skeletal-muscle AMP deaminase (AMP aminohydrolase, EC 3.5.4.6) with trypsin results in conversion of the enzyme into a species which over the pH range 6.5-7.1 exhibits hyperbolic kinetics at low K+ concentration even in the absence of ADP, but shows a 20% decrease in activity at saturating substrate concentration. Analysis by sedimentation-equilibrium techniques reveals the proteolysed enzyme to be homogeneous and to have a molecular mass of 222,000 Da, indicative of a trimeric structure with a subunit molecular mass of 72,000 Da, in contrast with the tetrameric structure of the native enzyme, composed of four 79,000-Da subunits. These observations suggest a role of the 7,000-Da fragment which is removed by proteolysis in the maintenance of the three-dimensional structure of the subunit that causes the enzyme at low K+ concentration to show homotropic positive co-operativity. Study of the influence of pH, isolated from that of K+, on the kinetics of AMP deaminase reveals a highly pH-dependent inhibitory effect by ATP which is completely absent at acid pH values and abruptly manifests itself just above neutrality. This phenomenon may have significance in the metabolism of exercising muscle, in connection with the pH-dependent interaction of AMP deaminase with the thick filament.