| Popis: |
By means of procedures for isolating subcellular structural components in a state free from cross-contamination, 5'-nucleotidase of the rat liver has been localized to the membranous component of the microsomes. Evidence has been obtained that suggests that a large fraction of the 5'-nucleotidase activity resides in the plasma membrane fragments that constitute a part of the population of the microsomal membranes. This enzyme is present in the lipoprotein residue of the plasma membrane that is not extractable with 0.9% NaCl solution. It is readily solubilized from the membrane with 0.5% solution of sodium deoxycholate. Isolated nuclei contained a negligible amount of 5'-nucleotidase activity, indicating that the substantial recovery of this enzyme previously reported in the nuclear fraction was due to contaminating microsomal membranes. 5'-Nucleotidase of the plasma membrane exhibited a pH optimum at about 7.5 when assayed in the absence of added metal ions. In the presence of 0.01 m Mg++, a second pH optimum appeared near 9.3. At pH 7.5 and in the presence of 0.01 m Mg++, a Km(amp) value of 2.2 x 10-5 m was obtained. |
