Postmortem hydrolysis

Niacin is present in tissues, and therefore in foods, iargeiy as the nicotinamide nucleotides. The postmortem hydrolysis of NAD(P) is extremely rapid in animal tissues, and it is likely that much of the niacin of meat (a major dietary source of the vitamin) is free nicotinamide. 

Two forms of vitamin B3, also known as niacin, are found in food [1,2] nicotinic acid and nicotinamide. In living tissues, nicotinamide is a moiety of the coenzymes nicotinamide adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP) in meat, it is found free because of the postmortem hydrolysis of NAD [1]. Nicotinamide is also a form used for food fortification [4]. Nicotinic acid is the prevalent vitamer in mature cereal grains nevertheless, it is unavailable due to its linkage to a number of polysaccharides (niacytin) and polypeptides (niacinogen) [1]. 

The colorimetric method is based on the hydrolysis of the substrate acetylthiocholine to acetate and thiocholine as performed by the cholinesterase. Thiocholine is then reacted with 5,5 -dithiobis(2-nitrobenzoic acid) (DTNB) to form a yellow anion (5-thio-2-nitrobenzoate). The latter is quantitated by spectrometric analysis at 405 nm, with the concentration being proportional to the cholinesterase activity in the given sample. Also for a few days postmortem the cholinesterase activity in different tissues is measurable. ... 

Important groups of endogenous enzymes that promote autolysis in postmortem seafoods include those originating from the gut, present in muscle tissues or synthesized and secreted in the extracellular matrix. The major autolytic reactions that cause deterioration of fresh seafood quality include proteolysis, glycolysis, nucleic acid (ATP) breakdown, and lipid hydrolysis/oxidation. 

It is reported that cathepsins B, D and L degrade myofibrillar proteins, while cathepsin H scarcely causes hydrolysis of these proteins (22,23), SDS-PAGE analyses show that a polypeptide with a molecular mass of 30 kDa is produced during the postmortem aging of meat (24-28), This was shown to be derived from a myofibrillar protein, troponin T. The peptide (APPPPAEVPEVHEEV) derived from troponin T is also reported to increase during the storage of beef (29). 

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