title: Kinetics of a nucleotide pyrophosphatase in the plasma membrane of the fat cell.
abstract: Fat cells from rat and rabbit hydrolyzed externally applied adenosine triphosphate at a rate of about 1.8 nmol times mg(-1) cells times min(-1) corresponding to about 0.3 mumol times mg(-1) protein tinus min(-1). Similar activities were found in cell homogenates. In purified adipocyte plasma membranes the rate of hydrolysis was about 1.8 mumol times mg(-1) protein times min(-1). The hydrolytic activity was dependent on divalent metal ions. Mg(2+), Mn(2+) and Ca(2+) gave highest activities. The activity was maximal at about equimolar concentrations of M(2+) and ATP. Km for MgATP was about 0.23 mM and for CaATP about 0.36 mM. Combinations of Mg(2+) and Ca(2+), or of Mg(2+), Na(+) and K(+) gave similar activities as did Mg(2+) only. At concentrations of 1 mM the following nucleotides were hydrolyzed with a decreasing rate: ATP > ITP > GTP > UTP = CTP. In isolated fat cells the beta-adrenergic drug isoproterenol and insulin slightly increased the rate of hydrolysis of external ATP, while the alpha-effector clonidine was inhibitory. The results suggest that a major portion of the ATP hydrolytic activity of the fat cell plasma membrane represents a nucleotide pyrophosphatase activity with access to externally applied ATP.
mesh_terms: D000251:Adenosine Triphosphatases / Q000378:metabolism*; D000273:Adipose Tissue / Q000166:cytology / Q000201:enzymology*; D000818:Animals; D000252:Calcium-Transporting ATPases / Q000378:metabolism; D002462:Cell Membrane / Q000201:enzymology; D006863:Hydrogen-Ion Concentration; D008274:Magnesium / Q000494:pharmacology; D008345:Manganese / Q000494:pharmacology; D011817:Rabbits; D051381:Rats; D013379:Substrate Specificity
chemical_list: D008345:Manganese; D000251:Adenosine Triphosphatases; D000252:Calcium-Transporting ATPases; D008274:Magnesium