To Determine the Quantity of Proteins
in Urine Sample Using Biuret Reaction^9
9.1 Theory
Healthy persons excrete urine that is largely free of proteins. The analysis of urine for
the presence of proteins is important for the diagnosis of renal, cardiac, and thyroid-
related diseases. Several physiological and pathophysiological mechanisms cause
the urinary excretion of proteins. The normal protein excretion in urine is about
30 – 150 mg/day. About one third of the total urinary protein is albumin, and the
remaining includes many small globulins. Small proteins arefiltered readily in
filtrate but are reabsorbed downstream by proximal tubular cells. The small amount
of albuminfiltered through the glomeruli is mostly absorbed by proximal tubular
cells and degraded by lysosomal enzymes into fragments, which are returned to
circulation in same way as that for small molecular weight proteins. The presence of
excess proteins in urine often causes the urine to becomefoamy, but other conditions
like presence ofbilirubinin the urine (bilirubinuria), pneumaturia (air bubbles in the
urine), or drugs may also make urine foamy. The presence of increased amounts of
proteins in the urine may indicate a serious disease problem or may appear before
any other clinical symptoms.
9.2 Specimen Requirements
A 12 h or 24 h urine specimen without any preservative is preferred. Use fresh
sample otherwise store at 2– 8 C for up to 48 h.
9.3 Principle
The protein in urine is precipitated with trichloroacetic acid (TCA), redissolved in
alkali, and measured colorimetrically using the Biuret reaction. Biuret reagent
contains alkaline CuSO 4 solution and sodium-potassium tartrate. The Cu2+ions
#Springer Nature Singapore Pte Ltd. 2018
V. Kumar, K. D. Gill,Basic Concepts in Clinical Biochemistry: A Practical Guide,
https://doi.org/10.1007/978-981-10-8186-6_9
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