Mediators, metabolic sensors

The most important enzymatic function of Ca " may be its action as a second messenger in the calmodulin-mediated activation of intracellular enzymes . Resting cells maintain a free Ca ion concentration in the cytosol of about 10" M by means of active Ca pumps and the sequestering of this ion by organelles (vesicular Ca " ). When a cell receives a stimulus (hormonal, electrical, etc.) channels are activated to let Ca flow in from the extracellular fluid, where it is present in millimolar concentrations, and the Ca " concentration in the cytosol rises to the 10" -10" M range. This stimulates certain metabolic processes by the activation of various enzymes. Most of these enzymes do not themselves bind Ca, but are activated by a Ca " -bound form of calmodulin (CaM), the ubiquitous intracellular sensor, of the increase in Ca " concentration found in virtually all cells. 

Several enzyme systems exist as cellular defense (detoxification) pathways against the chemically reactive metabolites generated by GYP metabolism (91,92,102,103). These include GST, epoxide hydrolase, and quinone reductase, as well as catalase, glutathione peroxidase, and superoxide dismutase, which detoxify the peroxide and superoxide by-products of metabolism. The efficiency of the bioinactivation process is dependent on the inherent chemical reactivity of the electrophilic intermediate, its affinity and selectivity of the reactive metabolite for the bioinactivation enzymes, the tissue expression of these enzymes, and the rapid upregulation of these enzymes and cofactors mediated by the cellular sensors of chemical stress. The reactive metabolites that can evade these defense systems may damage target proteins and nucleic acids by either oxidation or covalent modification. 

New developments in this area include uric acid sensors based on the mediation of urate oxidase by a novel redox polymer, poly(N-methyl-o-phenylenediamine) [145], and by the freely diffusing mediator 1-methoxy-5-methylphenazinium [146], continued research on the direct amperometric detection of NADH [147] and the use of redox mediators [148] for dehydrogenase enzymes, to allow practical sensors that exploit this large class of enzymes, and the use of cytochrome P450-modified glassy carbon electrodes as drug metabolism biosensors [149]. 

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