Purine nucleoside and purine base concentrations in bovine thyroid and plasma

Response of perfused lung and isolated pulmonary artery to adenosine

Adenosine (AD) has been reported to induce both pulmonary arterial constriction and dilation. We investigated the effect of AD using two complementary techniques. The isolated rat lung perfused with Earle's balanced salt solution containing albumin was used to measure pulmonary arterial (Ppa), venous, and double occlusion (microvascular; Pmv) pressure, and resistance changes. AD alone had no effect on Ppa, Pmv, or resistance at any dose tested (5 x 10(-7) through 10(-3) M). However, when Ppa was elevated by pretreatment with 5 x 10(-7) M norepinephrine (NE), then 10(-4) M AD lowered Ppa by 19.5 +/- 3.2% and Pmv by 6.0 +/- 6.1% and attenuated the increase in upstream resistance caused by NE. Higher doses of AD (10(-3) M) resulted in greater relaxation. In isolated segments from rat and guinea pig pulmonary lobar arteries, isometric force production in response to AD was measured as a percentage of the active isometric force produced in response to 10(-5) M NE (% NE contraction). No response was observed in rat pulmonary arterial rings for doses of AD less than 10(-6) M. In vessels with intact endothelium, 10(-6) M AD caused a slight increase in isometric tension (2.3 +/- 1.2% NE contraction; p less than 0.05), but 10(-4) M AD caused relaxation (-17.2 +/- 2.2% NE contraction; p less than 0.05), and 10(-3) M caused further relaxation (-61.5 +/- 5.0% NE contraction; p less than 0.05). In vessels without endothelium, only relaxation was observed. Isolated guinea pig arterial rings responded to AD with vasodilation similar to the results in the rat arterial rings. Results of this study show that AD primarily causes a direct dose-dependent relaxation of pulmonary arterial smooth muscle in both the isolated perfused lung and isolated arterial ring preparation.

Hypoxanthine and adenine metabolism in bovine thyroid tissue

Hypoxanthine was converted primarily to uric acid by thyroid tissue slices and homogenates with little inosine 5'-monophosphate formation while adenine was essentially all salvaged to adenosine 5'-monophosphate by similar tissue preparations. The ratio of hypoxanthine/guanine phosphoribosyltransferase activity to adenine phosphoribosyltransferase activity was 0.15 in the thyroid homogenates.

Adenosine kinase from bovine adrenal medulla

Adenosine kinase from bovine adrenal medulla was purified 1600-fold by using ammonium sulfate precipitation, gel filtration and affinity chromatography. Gel filtration yielded a relative molecular mass around 42000 and Michaelis constants were 0.2 microM for adenosine and 20 microM for MgATP. The enzyme showed a broad specificity for purine nucleoside triphosphate as phosphate donors. Both free Mg2+ and ATP were inhibitors. AMP was a competitive inhibitor with regard to adenosine and a non-competitive inhibitor versus MgATP, while ADP was a uncompetitive inhibitor with regard to adenosine and a non-competitive inhibitor versus MgATP. Adenosine kinase was strongly inhibited by the bis(adenylyl) polyphosphates Ap4A and Ap5A. These compounds inhibited the enzyme competitively versus MgATP (Ki = 0.06 microM for Ap4A and 0.4 microM for Ap5A) and uncompetitively with regard to adenosine. The results of the kinetic analysis suggest an ordered bi-bi mechanism, adenosine being the first substrate. The phosphorylation of adenosine was unaffected in the presence of vanadate ions.