The tyrosine is an aromatic amino acid, which can absorb light. It is formed form phenylalanine by the reaction catalyzed by phenylalanine hydroxylase. The phenylalanine is essential amino acid but tyrosine is not, provided the diet contains adequate quantities of phenylalanine. The reaction is not reversible, so tyrosine cannot replace the requirement for phenylalanine. Tyrosine has to sparing action, because if tyrosine is fed then need to phenylalanine in the body is decreased. Both the amino acids are ketogenic and at the same glycogenic. Both have immense metabolic role in the body, and are metabolized through the common pathways. The catabolism of tyrosine results in formation of acetoacetic acid, phenol, tyramine, epinephrine (adrenaline), non-epinephrine (nor adrenaline), thyroid hormones, and melanin in separate reactions.
Tyrosine is essential for protein synthesis and serves as a precursor for thyroxin, melanin, and catecholamine.
The amino acid tyrosine, found in chicken and also tuna fish, boosts brain chemicals that improve motivation and reaction time. The hereditary defective metabolic abnormalities of tyrosine lead to tyrosinosis, alcaptonuria (occhronosis), and albinism; which are described below.
The excess of tyrosine, when serum tyrosine levels are raised to 6-12 mg/dl as those of methionine, a condition called Tyrosinosis, affect the activities several enzymes and transport systems. In acute tyrosinosis, infants have diarrhoea, vomiting with a cabbage like odour and fail to thrive. In the absence of treatment, the death from liver failure takes place in 6-8 months. In chronic tyrosinosis, symptoms are similar but milder, and death occurs by 10 yrs of age. The treatment consists of a diet low in phenylalanine and tyrosine; also some times low even in methionine.
The other metabolic defect is tyrosinosis. Type II, wherein serum tyrosine level in raised to 4-5 mg/dl, is hepatic with eye and skin lesions, and some mental retardation. It is the only amino acid whose urinary concentration is raised whereas renal clearance and reabsorption of tyrosine fall within normal limits.
Another disorder, neonatal tyrosinosis, wherein blood levels of tyrosine and phenylalanine are raised, urinary level of tyrosine is raised. The treatment consists of a diet low in protein.
Melanins are polymers of tyrosine catabolites, catalyzed by tyrosine hydroxylase. Albinos with tyrosine hydroxylase: as negative lack all visual pigment and hair bulbs fail to convert added tyrosine to pigment; as positive have some visual pigment and hair bulbs convert tyrosine to black eumelanin in vitro.
Tyrosine forms epinephrine and nor epinephrine on getting converted in cells of neural origin through enzymes contained in adrenal medulla. The rate-limiting enzyme in the process of catecholamine biosynthesis is tyrosine hydroxylase. Tyrosine is also a precursor of the thyroid hormones. Free tyrosine can be iodinated by a reaction organification. Ascorbic acid (Vitamin C) is required in the degradation of tyrosine, and also in the synthesis of epinephrine from tyrosine.
It is available mainly in ragi, parboiled rice, cow pea, peas, soyabean, pumpkin leaves, spinach, colocasia, yam, ladies fingers, brinjal, grount nut, gingelly seeds, figs, strawberry, curd, milk.