Adenosine formation

The enzyme in the myocardium has recently attracted attention because of the possibility that adenosine is a physiological regulator of coronary blood flow (67) (adenosine is a potent coronary dilator). Most of the 5 -nucleotidase activity in rat heart is membrane bound, and a partially purified preparation has been obtained by extracting acetone powder preparations with deoxycholate (68). All 5 -nucleotides are hydrolyzed. The enzyme is strongly inhibited competitively by ATP (Ki 1.8 fxM). Whether this provides a regulatory mechanism for adenosine formation in the heart is not known. 

Figure 14 Reaction catalyzed by the radical SAM enzyme MiaB in 2-methylthio-N -isopentenyl-adenosine formation (R = ribose sugar). MiaB catalyzes the thiolation and methylation of the N -isopentenyl-adenosine precursor, apparently consuming two molecules of SAM in the process. SAH, S-adenosylhomocysteine.

Figure 6. a-and P-Adenosine formation (a) by dry heating adenine with salts and ribose with the formation of (b) a- and P-anomers among other products. 

Adenosine formation is accelerated under hypoxic conditions (Rudolphi et al. 1992), but it is metabohsed only after reperfusion. Now superoxide dismutase and catalase are insufficient to serve detoxification of the reactive oxygen species resulting from the biotransformation of hypoxanthine to xanthine and of xanthine to uric acid. While xanthine oxidase inhibitors as oxypurinol do protect against ischaemic damage (Helfman and Phillis 1989, Lin and Phillis 1992), adenosine deaminase inhibitors as trazodone (Sheid 1985) are expected to reduce the formation of hypoxanthine and xanthine, the substrates for xanthine oxidase. 

Since oxygen supply is usually unimpaired under physiological conditions, whereas oxygen demand can be quite variable, it is of critical importance to study the relationship of coronary flow and adenosine formation with altered levels of myocardial metabolic activity. Experiments were conducted in the open-chest rat in which cardiac work was increased by constriction of the thoracic aorta just above the diaphragm (Foley et al, 1978). The increased pressure work of the left ventricle was associated with an increase in the adenosine content of the myocardium (Figure 4). If the sum of adenosine and its degradative products, inosine and... 

They release adenosine diphosphate [58-64-0 (ADP) and thromboxane [57576-52-0] which results in vascular contraction and, indirectiy, in the formation of fibrin clot. Platelet transfusions are indicated for patients with thrombocytopenia, ie, a shortage of healthy platelets or thrombocytopathy, ie, platelet malignancy associated with spontaneous hemorrhages. 

ATP Adenosine triphosphate. Chemical energy generated by substrate oxidations is conserved by formation of high-energy compounds such as adenosine diphosphate (ADP) and adenosine triphosphate (ATP) or compounds containing the thioester bond. 

Several toxic compounds act by inhibiting the oxidation of carbohydrates or by inhibiting the formation of adenosine triphosphate (ATP), a molecule that... 

The structure and formation of ATP. (A) The chemical structure of adenosine triphosphate (ATP). "C" indicates carbon, "N" nitrogen, "O" oxygen, "H" hydrogen and "P" phosphorus. Note the negative charges on the phosphate groups (PO3 ). (B) ATP can be formed from adenosine diphosphate (ADP). 

The formation of a platelet aggregate requires the recruitment of additional platelets from the blood stream to the injured vessel wall. This process is executed through a variety of diffusible mediators which act through G-protein-coupled receptors. The main mediators involved in this process are adenosine diphosphate (ADP), thromboxane A2 (TXA2), and thrombin (factor Ila). These mediators of the second phase of platelet activation are formed in different ways. While ADP is secreted from platelets by exocytosis, the release of TXA2 follows its new formation in activated platelets. Thrombin can be formed on the surface of activated platelets (see Fig. 2). 

Escherichia coli Adenine and adenosine are inhibitory74 and the synthesis of thiamine can be derepressed by culture in their presence.13,75 adth- Mutants are known.76 [l4C]Formate incorporates at C-2 of pyramine without dilution of molar activity. Glycine labeled with stable isotopes was fed to E. coli and the pyramine was analyzed by mass spectrometry. The two carbon atoms of glycine separated during the biosynthesis. The carboxyl was found12 at C-4, and the C-N fragment was the precursor of C-6-N-1. In conclusion, it is beyond doubt that pyramine synthesis follows the AIR pathway in E. coli. 

Nanninga, L.B. Mommaerts. C.R. (1960). Studies on the formation of an enzyme substrate complex between myosin and adenosine-triphosphate. Proc. Natl. Acad. Sci. USA 46, 1155-1166. 

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