Dietary sources

All protein-containing foods provide Tyr. Relatively Tyr-rich protein comes with soy (55 mg/g protein), cow milk (4ft mg/g), and eggs (41 ing-'g). The Tyr content of flesh foods (beef, pork, chicken, and lish) is around 34 mg/g. The Tyr content of grain protein ranges widely. At the high end is corn (maize, 41 mg/g): at the low end is rye (20 mg/g). Wheat, oats, and rice as well as legumes contain around 30 mg/g. Typical Tvr iniakc of American adults may be 38 mg/kg body weight or more (Gibson et al. 2002).

Tyrosine in foods may be degraded by spoilage or processing. Bacterial EC4.1.1,25 tyrosine decarboxylase generates tyramine. Some fermented foods, especially cheeses, contain significant amounts of tyramine. Catechol oxidase (EC 1.10.3.1 ) in mushrooms, apples, potatoes, and other fresh plant foods causes newly cut surfaces to darken due to the oxidation of Tyr to monohydric and o-dihydric phenols.

Digestion and absorption

As food passes through the digestive tract, gastric and pancreatic enzymes continue protein breakdown, many of them cleaving peptide bonds between specific amino acids and Tyr.

I he sodium-independent transport system biv ' ferries Tyr across the intestinal brush border membrane in exchange for another neutral amino acid. This system consists of the light chain UAT1 (SLC7A9)and a heavy chain rUAT (SLC3AI ) glycoprotein that anchors the complex to the membrane (Verrey et al, 1999; Mizoguchi et al. 2001 ).

Brush border membrane

Baso lateral membrane

Capillary endothelium

Brush border membrane

Baso lateral membrane

Capillary endothelium

Figure H.37 Intestinal jhsorpnort of L-tyrosine

The T-type amino acid transporter 1 (TATI) facilitates diffusion across the basolateral membrane of enterocytes in jejunum, ileum, and colon (Kim et al,, 20011. Additional transport capacity is provided by the 4K2-glycoprotein-anchored exchanger I .AT2 (Rossier eta!., 1999: Rajan el rif., 2000).

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