Abstract
1. 1. Isolated rat liver nucleoli catalyze formation of P i with both ribo- and 2-deoxyribonucleoside diphosphates as substrates. Nucleolar GDPase activity is maximal at pH 7.2 to 7.8 and 40°; it is stimulated maximally by magnesium or manganese. The principal products of nucleolar GDPase are GMP and P i; no evidence has been obtained for a contribution to this activity by polynucleotide phosphorylase. 2. 2. Nucleoli also contain glucose-6-phosphatase and inorganic pyrophosphatase. Although such a content of phosphatases is analogous to that of microsomes, the absence of microsomes in the isolated nucleoli, and the non-identity of the nucleolar phosphatases with microsomal phosphatases, has been established by several means. 3. 3. The results of this work verify previous histochemical demonstrations of nucleoside diphosphatase activity in liver and hepatoma nucleoli.
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