Normal protein in urine
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Normal Protein in Urine: An Overview
Introduction to Urinary Proteins
Normal urine contains a variety of proteins, albeit in trace amounts. These proteins originate from different sources, including plasma proteins that pass through the glomeruli, proteins from the urinary tract, and in males, seminal proteins from the genital tract1 2. The presence of these proteins in urine is a normal physiological process and does not necessarily indicate any pathology.
Types of Proteins in Normal Urine
Plasma Proteins
Plasma proteins enter the urine from the blood through the glomeruli. Most plasma proteins, except for fibrinogen and β-lipoprotein, can be detected in normal urine1. The daily excretion of these proteins is typically around 40 mg, with variations ranging from 25 to 90 mg per day2. The albumin/globulin ratio in urine is reversed compared to blood, with globulins being more prevalent4.
Urinary Tract Proteins
Proteins from the urinary tract, such as the mucoprotein described by Tamm and Horsfall, are also present in normal urine. These proteins originate from the glands or shed cells of the urinary tract, from the kidney tubules downwards1 2.
Seminal Proteins
In males, proteins from the seminal plasma, particularly from the prostate and bulbo-urethral glands, contribute to the protein content in urine1.
Quantitative Measurement of Urinary Proteins
Biuret Method
The biuret method is a reliable technique for quantifying protein in normal urine. This method involves precipitating proteins with ethanolic phosphotungstic acid, followed by measurement using biuret reagent. The sensitivity of this method is 0.5 mg/100 ml, and it has a coefficient of variation of 4.2%3.
Pyrogallol Red-Molybdate Complex Method
Another method for determining urinary protein involves the use of a pyrogallol red-molybdate complex. This method is applicable to both automated and manual measurements and provides a normal range for urinary protein from 28 to 141 mg/day5.
Electrophoretic and Proteomic Analysis
Electrophoretic Techniques
Electrophoretic analysis has been used to study the protein composition of normal urine. This method requires concentrating the urine samples significantly and has shown that normal urine contains proteins with mobilities similar to those in serum, although the separation of globulin components is less defined2 4 8.
Proteomic Techniques
Proteomic analysis using techniques like two-dimensional polyacrylamide gel electrophoresis (2D-PAGE) and mass spectrometry has identified a wide range of proteins in normal urine. These include transporters, adhesion molecules, enzymes, and cell signaling proteins. Combining different isolation techniques enhances the identification of these proteins6.
Clinical Implications
The presence of proteins in normal urine is a well-documented phenomenon and does not necessarily indicate disease. However, significant deviations from the normal range can be indicative of pathological conditions such as proteinuria, which is a marker for kidney disease9. Accurate measurement and analysis of urinary proteins are crucial for diagnosing and monitoring renal health.
Conclusion
Normal urine contains a variety of proteins from plasma, the urinary tract, and, in males, the genital tract. Quantitative and qualitative analyses of these proteins are essential for understanding renal physiology and identifying potential pathologies. Advanced techniques like electrophoresis and proteomics provide detailed insights into the protein composition of normal urine, aiding in the accurate assessment of renal function.
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Most relevant research papers on this topic
THE PROTEINS OF NORMAL URINE
Normal urine contains traces of various proteins from plasma, urinary tract, and male seminal sources.
The Proteins of Normal Urine *
Urinary proteins from blood and urinary tract can be clearly distinguished using immunochemical techniques.
A biuret method for determination of protein in normal urine.
The biuret method accurately measures protein in normal urine without interference from drugs or pigments, making it suitable for monitoring clinical trials of new drugs for detecting nephrotoxicity.
Highly CitedThe Plasma Proteins in Normal Urine
Electrophoretic fractionation of normal urine does not provide satisfactory information on the types and amounts of plasma proteins present, and immunochemical methods offer better possibilities for identifying individual proteins.
Urinary protein as measured with a pyrogallol red-molybdate complex, manually and in a Hitachi 726 automated analyzer.
This new method for determining urinary protein within 10 minutes at 37 degrees C provides accurate results with low errors and a normal range of 28 to 141 mg/day.
Highly CitedProteomic analysis of normal human urinary proteins isolated by acetone precipitation or ultracentrifugation.
Combining acetone precipitation and ultracentrifugation techniques enhances protein identification in human urine, offering potential for studying renal physiology and pathophysiology and identifying biomarkers of renal disease.
In Vitro StudyHighly CitedUrinary total protein estimation--fact or fiction?
Urinary total protein estimation may not accurately detect proteinuria, and urinary albumin may be a more accurate method for determining protein content in urine.
Observational Study