Thiolase from pig heart shows in dilute salt solutions the following hydrodynamic properties: s°20,w= 7.88 S D°20,w= 4.2 × 10−7 cm2/sec = 0.731 ml/g From these data a molecular weight of 169,000 was calculated which is in good agreement with the value of 168,000 obtained by the Archibald method.In the presence of 5 M guanidine‐HCl the enzyme dissociates into subunits with a molecular weight in the range of 39,000–44,000 as determined by use of the Archibald method, sedimentation equilibrium analysis and combination of sedimentation velocity with viscosity and diffusion measurements.The sedimentation coefficient and the molecular weight of the subunits in 5 M urea (s°20,w= 1.86 S, mol. wt. = 43,700) and in 70% formic acid (s°20,w= 1.73 S, mol. wt. = 41,700) are comparable to those obtained in the presence of guanidine‐HCl.Treatment of the enzyme with a large excess of succinic anhydride caused a splitting into protein particles with a molecular weight of approximately 92,000, corresponding to half the size of the native enzyme.Quantitative determinations of the NH2‐terminal amino acid residues by reaction with dinitrofluorobenzene and by Edman degradation yielded 4 valine residues p
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