biochemical tests for proteinuria

[chris 908]

why sulphosalycilic acid is used for testing the presence of proteins in urine?

 

in heller's test why albumin only is being tested?what is the underlying principle for it being used to detect albumin in urine specifically?

[CharonY]

It is an unspecific test for proteins and based on the denaturation of proteins in presence of a sufficiently strong acid.

[chris 908]

why Sulphosalycilic acid is selected for non specific testing....whereas heller's test using HNO3 is indicated for testing albuminuria...has HNO3 got anythng to do specially with albumin....

[CharonY]

The precipitation mechanisms are slightly different, but results are similar. I.e. nitric acid will not specifically precipitate a particular protein. There is a variation of the SSA method that allows a colorimetric and thus more quantitative method. The reason why it is used for albumin (+globulin) testing is that normally that these two are the highest abundant proteins. However, if there are other proteins present in significant amount, they will interfere with the test.

 

The methods are as such not terribly selective, other than that some protein species are more prone to precipitate than others. Generally, the SSA method is reported to be slightly more sensitive than Heller but otherwise yield very similar results (each are somewhat prone to a different set of interfering substances, such as e.g. certain antibiotics, and are sometimes used in tandem).

[chris 908]

thanks....but if the purpose was simply to acidify the medium,aren't there many better acids than ssa??......or is it terribly sensitive to the slightest amount.???

do albumin has lower specific gravity than other proteins that might be found present in urine??(in hellers test nitric acid is added via the test tube walls to make a seperate layer of acid above the test sample to effect the formation of a white ring at the junction if the sample contains albumin)

[CharonY]

Those acids work well at room temperature. Others such as acetic acid are also used but require heating. And some other acids are more likely to hydrolyse proteins too fast resulting in soluble breakdown products instead of visible turbidity. I assume one could adapt it to quite a few other precipitants, but these are the commonly used ones. Another thing to note is that urine also contains other components and varies a bit in pH. Care must be taken that the normal constituents do not precipitate either, otherwise one would not be able to distinguish between presence or absence of proteins.

 

If you mix a large amount of proteins into urine and run Heller, you will get positive result even in the absence of albumin. I.e. if you have leaking of other proteins in urine in significant amounts (which would be bad) Heller would still give a positive signal.

The ring is less a property of albumin but to having a interface (between urine and the nitric acid) at which the proteins precipitate.

Edited April 29, 2014 by CharonY