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108

 Functional Assessment of Urinary Neuro-biogenic Amines—A COMPREHENSIVE GUIDE

mechanisms, redox regulation, and func-

tional significance.

Antioxidants & redox signaling

vol. 15 (3) p.

691-718

Online Mendelian Inheritance in Man

(OMIM)

Diazepam binding inhibitor; DBI

Accessed 26 December 2014

Pearl, Phillip L; Hartka, Thomas R; Cabalza,

Jessica L; Taylor, Jacob; Gibson, Michael

K (2006)

Inherited disorders of GABA metabolism.

Future neurology

vol. 1 (5) p. 631-636

Taneera, J; Jin, Z; Jin, Y; Muhammed, S J;

Zhang, E et al. (2012)

γ-Aminobutyric acid (GABA) signalling in

human pancreatic islets is altered in type

2 diabetes.

Diabetologia

vol. 55 (7) p. 1985-94

Wang, Mingde (2011)

Neurosteroids and GABA-A Receptor

Function.

Frontiers in endocrinology

vol. 2 p. 44

Tyrosine

Tyrosine is a non-essential amino

acid that may be acquired from dietary

proteins or synthesized from dietary

phenylalanine, an essential amino acid.

Tyrosine is the precursor for the cate-

cholamine neurotransmitters dopamine,

norepinephrine and epinephrine; tyro-

sine availability may affect the synthe-

sis of these catecholamines. Tyrosine is

also the precursor for thyroxine (thyroid

hormone) and melanin (skin pigment).

The proportion of dietary tyrosine that

enters systemic circulation from the

diet is controlled by the enzyme tyro-

sine aminotransferase (TAT) in liver and

kidney. Tyrosine may be considered a

conditionally essential amino acid if its

precursor phenylalanine is deficient or

in patients with phenylketonuria (PKU),

an inherited disorder of phenylalanine

metabolism.

Effects:

Decreased

tyrosine levels may im-

pair both spatial recognition memory

and mental performance. Low tyrosine

levels may increase irritability scores

during psychological challenge testing.

Human studies of short-term dietary

tyrosine depletion indicate that low ty-

rosine levels may increase prolactin hor-

mone levels and may lower mood and

energy levels. Tyrosine depletion has

also been reported to decrease observa-

tional clinical ratings for manic symp-

toms in patients. Long-term depletion

of dietary tyrosine may result in cate-

cholamine neurotransmitter depletion,

which may affect mood. Low tyrosine

levels have been associated with de-

creases in body temperature and thy-

roid function. Elevated glucocorticoid,

insulin, glucagon, or tryptophan levels

may induce liver tyrosine aminotrans-

ferase (TAT), which may decrease tyro-

sine levels. Conversion of phenylalanine

to tyrosine requires tetrahydrobiopterin

(BH4). Tyrosine levels may decrease if

phenylalanine is deficient or in patients

with PKU. Human studies indicate that

Tyrosine supplementation may improve

cognition and performance under stress-

ful conditions.

Excess

tyrosine levels may occur

due to heritable enzyme defects, liver

disease, or supplementation. Elevated

tyrosine levels may interfere with med-

ications such as monoamine oxidase

inhibitors (MAOIs), thyroid hormone

replacement and L-dopa replacement.

Migraine headaches and hyperthyroid

conditions may be exacerbated by ele-

vated tyrosine levels. High levels of tyro-

sine may result in elevated levels of the

trace amine tyramine.