Adenosine, generally deaminase

Unlike classical neurotransmitters, adenosine does not have a rapid synaptic uptake system (as for the biogenic amines), and its chemical inactivation system is not as rapid as for the transmitter acetylcholine, for example. Adenosine may be metabolized extracellularly and inactivated with respect to the ARs in a more general fashion by the widespread enzymes adenosine kinase (AK, to produce AMP) and adenosine deaminase (AD, to produce inosine). Both AMP and inosine are only weakly active at ARs, depending on the subtype. 

Adenosine is not a classical neurotransmitter because it is not stored in neuronal synaptic granules or released in quanta. It is generally thought of as a neuromodulator that gains access to the extracellular space in part from the breakdown of extracellular adenine nucleotides and in part by translocation from the cytoplasm of cells by nucleoside transport proteins, particularly in stressed or ischemic tissues (Fig. 17-2C). Extracellular adenosine is rapidly removed in part by reuptake into cells and conversion to AMP by adenosine kinase and in part by degradation to inosine by adenosine deaminases. Adenosine deaminase is mainly cytosolic but it also occurs as a cell surface ectoenzyme. 

Pharmacokinetics Pegademase bovine is rapidly absorbed following intramuscular administration of Adagen, plasma adenosine deaminase activity generally normalizes after 2 to 3 weeks of weekly intramuscular injections. The half-life of pegademase is 48 to 72 hours. 

Leucine aminopeptidase is interesting in that its active site contains two zinc atoms which together bind and activate the water molecule [74]. Despite this enzyme containing a dinuclear metal center at its active site, its mechanism, and specifically its mode of proton transfers reactions, appear to follow the general theme established by thermolysin and carboxypeptidase Adenosine deaminase and other members of the family of nucleoside and nucleotide deaminases utilize zinc-bound water as the catalytic nucleophile to displace ammonia from the 6-position of purines or the 4-position of pyrimidines and in all cases display inverse solvent deuterium isotope effects ranging from 0.3 to 0.8 on fec/Kni [75-80]. These effects are reminiscent of those observed for metallopro-teases and have their origins, like those of the proteases, in fractionation factors for the protons of the bound water that are less than one. 

Fig. 11. The structure of the metal ion site in adenosine deaminase. The drawing is adapted from the data in Ref. 105. The Zn serves to activate a water molecule for nucleophilic attack while also orienting a carboxylate ligand to serve as a general base to deprotonate the water molecule.

Structural analogues of the formycins have been reported in a paper describing a general route to 2-glycosylated pyrimidinones (see Scheme 17). Treatment of formycin with iViV-dimethylformamide dineopentyl acetal gave the 2,5 -cyclo-nucleoside as one of the products, and 2, 3 -0-isopropylideneformycin was converted into the 4,5 -cyclonucleoside by way of the 5 -toluene-p-sulphonate. Adenosine deaminase caused deamination of the 2,S -cyclonucleoside only, indicating that the a //-conformation of a substrate is utilized by the enzyme. 

The mutant strain (S0 405) lacks the enzyme adenylosuccinate lyase, required for novo purine biosynthesis as well as for the conversion of s-AMP to AMP. This strain therefore has a specific requirement for an adenine compound and a general purine requirement. As the strain lacks nucleoside phosphorylase and adenosine deaminase the pathways for nucleoside utilization via free bases are ruled out. 

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