Proteins in Other Body Fluids

Proteins in Other Body Fluids In addition to plasma, proteins are found in several other body fluids and tissues, including urine, CSF, amniotlc fluid, saliva, and feces. 

In addition, the proportions of individual proteins excreted in the urine depend on the extent of their reabsorption by the proximal renal tubules, which is also inversely proportional to molecular size. The reabsorption of low molecular weight proteins involves binding of the protein to specific receptors on the tubule epithelial cells, uptake into the cells by pinocytosis, fusion of uptake vacuoles with lysosomes, and hydrolysis of the protein by lysosomal enzymes, followed by return of the constituent amino acids to the body pool. Very little of the total excreted urinary protein normally consists of small proteins. Only a small amount of protein is excreted normally (20 to 150mg/dL), and most of it is albumin because of its concentration—it is not completely removed from the filtrate by the tubular cells. The remainder is almost entirely the Tamm-Horsfall protein uromucoid, a constituent of urinary casts, probably secreted by the distal tubules. 

In addition to hemorrhage, proteinuria (increased amounts of protein in urine) occurs with (1) increased glomerular 

The protein excretion rate is ordinarily determined from a 24-hour urine collection because random specimens vary considerably in protein concentration. However, a more convenient alternative is to determine, in random samples, the ratio of protein to creatinine (protein-creatinine index). Creatinine concentration is relatively constant in any one subject, and the index correlates well with the 24-hour total excretion of protein. Total protein determination, however, does not discriminate between individual proteins, and when 24-hour excretion or protein-creatinine index values are near normal, estimation of one or two individual representative proteins is preferred. When protein excretion 

An additional clinical entity, microalbuminuria, is recognized as a strong predictor of impending nephropathy in type I diabetic patients (see Chapter 25). 

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