Benders'dictionary of nutrition and food technology

394and human milk proteins, which are used with 90–100% efficiency(BV = 0.9–1.0) when fed at low levels in the diet (4%),but not when fed at high levels (10–15%).protein retention efficiency (PRE) A measure of proteinquality.protein score A measure of protein quality based on chemicalanalysis.protein, second class See protein, first class.protein turnover See half-life (1).proteoglycans See glycoproteins.proteolysis The hydrolysis of proteins to their constituent aminoacids, catalysed by alkali, acid or enzymes.proteomics Identification of all the proteins present in a cell,tissue or organism. The proteome cannot be predicted from thetranscriptome, because of post-translational modifications suchas glycosylation, esterification and phosphorylation that areinvolved in the synthesis of many proteins.proteoses Partial degradation products of proteins; soluble inwater. The stages of breakdown are protein → proteoses →peptones → polypeptides → oligopeptides → amino acids.Proteus Genus of flagellate and highly motile rod-like gramnegativebacteria, common in intestinal flora. Some species arepathogenic.prothrombin Protein in plasma involved in coagulation of blood.Prothrombin time is an index of the coagulability of blood (andhence of vitamin k nutritional status) based on the time takenfor a citrated sample of blood to clot when calcium ions andthromboplastin are added.protoalkaloids Amines synthesised by decarboxylation of aminoacids.See also alkaloids.protogen See lipoic acid.proton pump Enzyme (H + /K + ATPase, EC 3.6.1.36) in parietal(oxyntic) cells of gastric mucosa that causes secretion of gastricacid; acts by exchanging H + and K + across the cell membrane.Irreversible inhibitors (e.g. lansoprazole, pantoprazole andomeprazole) are used in the treatment of gastric ulcers andhiatus hernia.protopectin See pectin.protoporphyrin The iron-free precursor of haem. Normallypresent in red blood cells in low concentrations, an increasedconcentration is an early index of iron deficiency. Also increasedby lead toxicity.See also porphyrin.proving The stage in breadmaking when the dough is left to rise.