RESUMO
Two active isoforms of bovine liver phosphorylase with distinct subunit composition have previously been purified (Cámara Artigas, A., Barón, C. and Parody-Morreale, A. Prot. Express. Purif. 1994, 5, 157), one showing three SDS-PAGE polypeptide bands (molecular mass = 97, 55 and 40 kDa) and the other showing just one (molecular mass = 97 kDa). A molecular mass of 200 kDa has been determined for the native enzymes by gel filtration. Amino acid analyses have been performed in both cases, giving the same results which are similar to those obtained with other phosphorylases. SDS-PAGE experiments at different concentrations of the three-band enzyme have suggested a 1:1:1 stoichiometry between the polypeptides. The pyridoxal-5'-phosphate site is located in the 55 kDa polypeptide and the phosphorylation site in the 40 kDa one. These polypeptides can be generated from the three-band enzyme by tryptic attack in the presence of glycogen without loss of enzyme activity. In the absence of glycogen, 55 kDa and 38 kDa polypeptides are generated, with a significant decrease in activity. We conclude that the three-band enzyme is a dimer composed of an intact monomer and a broken one. The lyotropic salt activation site of the enzyme is near the amino terminal group.
Assuntos
Fígado/enzimologia , Fosforilases/química , Aminoácidos/análise , Animais , Sítios de Ligação , Bovinos , Eletroforese em Gel de Poliacrilamida/métodos , Peso Molecular , Fragmentos de Peptídeos/química , Fosforilases/metabolismo , Conformação Proteica , Desnaturação Proteica , Fosfato de Piridoxal/análise , Sulfatos/química , Sulfatos/metabolismo , Tripsina/metabolismoRESUMO
The procedures for the purification of two forms of bovine liver glycogen phosphorylase b are described. Both forms showed a single band in nondenaturing gel electrophoresis. Gel electrophoresis in the presence of sodium dodecyl sulfate produced a single-band pattern for one of the enzyme forms (phosphorylase b1) and a triple-band pattern for the other (phosphorylase b3). Molecular weights associated with these bands were 97 kDa in the first case and 97, 55, and 40 kDa in the second. The yield from 1 kg of liver was approximately 10 mg for phosphorylase b1 and 140 mg for phosphorylase b3. The specific activity was 40-44 U/mg in both cases. As phosphorylase b1 is composed of just one kind of monomer, it is a novel bovine liver phosphorylase b structure.
