Nicotine enzymatic degradation

Either acid or alkaline hydrolysis can be applied, converting nicotinamide to nicotinic acid. Alkaline hydrolysis releases also the unavailable vitamers providing the estimation of the total niacin content. Acid hydrolysis, instead, is slower than alkaline hydrolysis therefore the former is usually coupled with enzymatic digestion by using takadiastase, papain, and clarase. Extraction with water and dilute sulfuric or hydrochloride acid has been applied to release the vitamers from the matrix without degrading nicotinamide [598]. 

Only tryptophanuria due to (presumed) defects in tryptophan degradation via the kynurenine pathway is considered here. Tryptophan transport defects, renal and/or intestinal (Hartnup disorder, blue-diaper syndrome) are discussed in Sect. 13.4. The symptomatology of reported cases is caused by nicotinic acid deficiency, one of the products of the said pathway. The postulated enzymatic defects have not been confirmed by in vitro investigations [7-9]. 

The first enzyme in the kynurenine pathway of tryptophan degradation, liver tryptophan pyrrolase, is inducible by tryptophan and inhibited by reduced nicotinamide adenine dinucleotide. In the case of dietary nicotinic acid deficiency the kynurenine pathway becomes important for nicotinic acid synthesis. Nicotinic acid deficiency causes pellagra. Blocks in the kynurenine pathway because of cofactor deficiencies and/or enzymatic defects result quite often in pellagra-like symptoms. 

Of the two pyridine nucleotide coenzymes, NAD is present mainly as the oxidized form in the tissues, whereas NADP is principally present in the reduced form, NADPH2. There are important homeostatic regulation mechanisms which ensure and maintain an appropriate ratio of these coenzymes in then-respective oxidized or reduced forms in healthy tissues. Once converted to coenzymes within the cells, the niacin therein is effectively trapped, and can only diffuse out again after degradation to smaller molecules. This implies, of course, that the synthesis of the essential coenzyme nucleotides must occur within each tissue and cell type, each of which must possess the enzymatic apparatus for their synthesis from the precursor niacin. Loss of nicotinamide and nicotinic acid into the urine is minimized (except when the intake exceeds requirements) by means of an efficient reabsorption from the glomerular filtrate. 

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