Proteins processed, nutritional consequences

D-Amino Acids in Processed Proteins Their Nutritional Consequences... 

Carbonaro, M., Bonomi, F., lametti, S., Cappelloni, M., and Camovale, E., Aggregation of proteins in whey from raw and heat-processed milk Formation of soluble macroaggregates and nutritional consequences, Lebensm. Wiss. TechnoL, 31, 522, 1998. 

The nutritive quality of any protein depends on three factors amino acid composition, digestibility, and utilization of the released amino acids. Racemization brought about by processing can impair the nutritive value of proteins by (1) generating non-metabo-lizable forms of amino acids (some D-enantiomers), (2) creating peptide bonds inaccessible to proteolytic enzymes, possibly restricting release of essential amino acids (3) toxic action (or interaction) of specific D-enantiomers. Little is known concerning the nutritional consequences of human consumption of racemized proteins. 

Not all calcium present in the diet is absorbed by the small intestine and mechanisms are present to ensure only amounts appropriate to body needs are absorbed. These processes are complex and involve the interaction of special transport protein, vitamin D and parathormone. Thus, abnormalities of calcium metabolism may result from many different disease processes. Diseases affecting the bowel may prevent normal absorption, diseases of the parathyroid gland may result in inappropriate levels of parathormone for calcium requirement and a nutritionally inadequate diet may cause vitamin D deficiency with consequent disordered calcium absorption. 

Fig. 39.2 Suggested effects leading to neuroprotective activity of hupA in AD. HupA is considered to modulate the APP processing by inducing the activity of a-secretase. The increased activity of a-secretase causes enhancement of nutritional APP (sAPPa) and consequently inhibition the A[i pathway. These APP-modulating effects are mediated through Ml muscarinic receptor-mediated PKC-dependent cascade and Trk receptor-mediated ERK/MAPK-dependent cascade and caused by increased levels of ACh and NGF in the synaptic junction. Increased release of sAPPa and decreased A(3 release from APP followed by inhibition of A[i fibril formations affects the regulation of the expression of apoptotic proteins, attenuates oxidative stress, and allows the mitochondria, respiratory chain, and TCA cycle to function normally. APP amyloid precursor protein, PKC protein kinase C. ERK extracellular signal-regulating kinase, MAPK mitogen-activated protein kinase, TCA tricarboxylic acid cycle (citric acid cycle)...

In heat treated or stored food products several amino acids are not fully available because of derivatization or crosslinking reactions. Since 30 years furosine is known as a useful indicator of early Maillard reaction which is applied in food science, nutrition and medical biochemistry. Recently more sensitive analytical methods for furosine determination are available which have again increased the attractivity of this important indicator. Lately, N -carboxymethyllysine (CML) became available as another marker of special interest, because CML is a more useful indicator of the advanced heat damage by Maillard reaction than furosine. In addition, CML has the advantage to indicate reactions of lysine with ascorbic acid or ketoses such as fructose. Indicators for protein oxidation of sulfur amino acids are methionine sulfoxide and cysteic acid. An established marker for cross-linking reactions is lysinoalanine, which also indicates protein damages due to processing under alkaline conditions. Other markers formed as a consequence of alkaline treatment are D-amino acids. 

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