Cells of the zona fasciculata and zona reticularis lack aldosterone synthase (CYP11B2) that converts corticosterone to aldosterone, and thus these tissues produce only the weak mineralocorticoid corticosterone. However, both these zones do contain the CYP17A1 missing in zona glomerulosa and thus produce the major glucocorticoid, cortisol. Zona fasciculata and zona reticularis cells also contain CYP17A1, whose 17,20-lyase activity is responsible for producing the androgens, dehydroepiandosterone (DHEA) and androstenedione. Thus, fasciculata and reticularis cells can make corticosteroids and the adrenal androgens, but not aldosterone.
Mannitol is a polyol that occurs in a wide range of living organisms, where it fulfills different physiological roles. Several pathways have been described for the metabolism of mannitol by bacteria, including the phosphoenolpyruvate-dependent phosphotransferase system (PST) and a M2DH-based catabolic pathway. The latter involves two enzymes, a mannitol-2-dehydrogenase (EC ) and a fructokinase (EC ), and has been identified in different bacteria, . , the marine Bacteroidetes Zobellia galactanivorans ( Zg ) which had recently gained interest to study the degradation of macroalgal polysaccharides. This protocol describes the biochemical characterization of a recombinant mannitol-2-dehydrogenase (M2DH) of Zobellia galactanivorans . The Zg M2DH enzyme catalyzes the reversible conversion of mannitol to fructose using NAD + as a cofactor. Zg M2DH activity was assayed in both directions, . , fructose reduction and mannitol oxidation.
Reversible reaction :
D-mannitol + NAD + ↔ D-fructose + NADH + H +
All energy yielding process are ultimately dependent upon enzymatically catalyzed redox reactions. The most important one for energy metabolism involve biological membranes with bound electron transport processes like photosynthesis and oxidative phosphorylation. Biological oxidation is the primary provider of energy for cellular anabolism, the reductive synthesis of metabolites, by furnishing mobile hydrogens, and phosporylating energy by combining hydrogens with oxygen to form water coupling this process to the production of ATP in the form of oxidative phosphorylation. Central to the oxidation-reduction processes are the vitamin B group containing coenzymes nicotinamide-adenine dinucleotide (NAD) and nicotinamide-adenine dinucleotide phosphate ( NADP , (C00006; oxidized form); NAD (C00003; oxidized form; not phosphorylated at the adenosine ribosyl C2 position). Being part of the appropriate enzymes the oxidized nicotinamide ring of NAD + or NADP + extracts a hydride (H: - ) from a wide variety of simple metabolites in a process known as dehydrogenation . The enzymes catalyzing the reduction of nicotinamide containing coenzymes are called dehydrogenases . In a typical reaction two hydrogen atoms (including their electrons) are removed from the substrate to produce the oxidized form of the donor. The fate of the two hydrogens differs: one hydrogen with two electrons (H: - ), a hydride ion, is transferred to the nicotinamide ring to produce reduced NADH or NADPH while the other hydrogen is released into solution as a free proton (H + ). The generic form of a redox reaction mechanism catalyzed by enzymes with NAD as cofactor is shown.