Nucleosidediphosphatase activity in plasma membrane of rat liver

Abstract

1. 1. The intracellular distribution of nucleosidediphosphatase activity has been investigated in rat liver with ADP and IDP as substrates and compared with those of reference enzymes. 2. 2. After differential centrifugation in 0.25 M sucrose, the distribution of the enzyme acting on IDP is similar to that of glucose-6-phosphatase; ADPase distribution is comparable to that of 5′-AMPase. 3. 3. Deoxycholate considerably increases nucleosidephosphatase activity on IDP, but has little effect when ADP is used as the substrate. The detergent does not affect the distribution of ADPase; on the contrary, significant changes of the IDPase distribution are observed if the test is performed with or without deoxycholate. 4. 4. Enzyme distribution curves have been determined after centrifugation in a sucrose gradient of a particle preparation obtained by centrifuging the homogenate at 3.7·10 6 × g· min . When granules are layered at the top of the gradient no clear distinction is seen between the enzyme distribution curves. When granules are deposited at the bottom of the gradient, a striking separation of the nucleosidediphosphatase distribution curves can be observed. The behaviour of IDPase is similar to that of glucose-6-phosphatase; ADPase behaves like 5′-AMPase. 5. 5. In purified plasma membrane preparations, ADPase and 5′-AMPase show a specific activity approx. 25 times higher than that fund in the homogenate. IDPase is only 5 times purer than in the homogenate. Some properties of the nucleosidediphosphatase associated with the plasma membrane are given. 6. 6. The results are discussed with respect to the presence in rat liver of two nucleosidediphosphatases, one associated with the plasma membrane and the other with endoplasmic-reticulum membranes.