Page 333 - Color Atlas of Biochemistry

324 Tissues and organs

Urine Modified amino acids, which occur in special
proteins such as hydroxyproline in collagen
and 3-methylhistidine in actin and myosin,
A. Urine
can be used as indicators of the degradation
Water and water-soluble compounds are ex- of these proteins.
creted with the urine. The volume and com- Other components of the urine are conju-
position of urine are subject to wide variation gates with sulfuric acid, glucuronic acid, gly-
anddependon foodintake, body weight, age, cine, and other polar compounds that are
sex, and living conditions such as tempera- synthesized in the liver by biotransformation
ture, humidity, physical activity, and health (see p. 316). In addition, metabolites of many
status. As there is a marked circadian rhythm hormones (catecholamines, steroids, seroto-
in urine excretion, the amount of urine and its nin) also appear in the urine and can provide
composition are usually given relative to a 24- information about hormone production. The
hour period. proteohormone chorionic gonadotropin (hCG,
A human adult produces 0.5–2.0 L urine mass ca. 36 kDa), which is formed at the onset
per day, around 95% of which consists of of pregnancy, appears in the urine due to its
water. The urine usually has a slightly acidic relatively small size. Evidence of hCG in the
pH value (around 5.8). However, the pH value urine providesthe basisfor an immunological
of urine is strongly affected by metabolic sta- pregnancy test.
tus. After ingestion of large amounts of plant The yellow color of urine is due to uro-
food, it can increase to over 7. chromes, which are related to the bile pig-
ments produced by hemoglobin degradation
(see p. 194). If urine is left to stand long
B. Organic components
enough, oxidation of the urochromes may
Nitrogen-containing compounds are among lead to a darkening in color.
the most important organic components of
urine. Urea, which is mainly synthesized in
the liver (urea cycle; see p.182), is the form in C. Inorganic components
which nitrogen atoms from amino acids are The main inorganic components of the urine
2+
+
+
2+
excreted. Breakdown of pyrimidine bases also are the cations Na ,K ,Ca ,Mg ,and NH 4 +
–
2–
2–
produces a certain amount of urea (see and the anions Cl ,SO 4 ,and HPO 4 ,as well
+
p. 190). When the nitrogen balance is con- as traces of other ions. In total, Na and Cl –
stant, as much nitrogen is excreted as is taken represent about two-thirds of all the electro-
up (see p. 174), and the amount of urea in the lytes in the final urine. Calcium and magne-
urine therefore reflects protein degradation: sium occur in the feces in even larger quanti-
70 g protein in food yields approximately ties. The amounts of the various inorganic
30 g urea in the urine. components of the urine also depend on the
Uric acid is the end product of the purine composition of the diet. For example, in
metabolism. When uric acid excretion via the acidosis there can be a marked increase in
kidneysisdisturbed, gout can develop(see the excretion of ammonia (see p. 326). Excre-
+
+
2+
p. 190). Creatinine is derived from the muscle tion of Na ,K ,Ca ,and phosphatevia the
metabolism, where it arises spontaneously kidneysissubject to hormonal regulation (see
and irreversibly by cyclization of creatine p. 330).
and creatine phosphate (see p. 336). Since
the amount of creatinine an individual ex-
cretes per day is constant (it is directly pro- Further information
portional to muscle mass), creatinine as an Shifts in the concentrations of the physiolog-
endogenous substance can be used to mea- ical components of the urine and the appear-
sure the glomerular filtration rate. The ance of pathological urine components can be
amount of amino acids excreted in free form used to diagnose diseases. Important exam-
is strongly dependent on the diet and on the ples are glucose and ketone bodies, which are
ef ciency of liver function. Amino acid deriv- excreted to a greater extent in diabetes
atives are also found in the urine (e. g., hippu- mellitus (see p. 160).
rate, a detoxification product of benzoic acid).

Koolman, Color Atlas of Biochemistry, 2nd edition © 2005 Thieme
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