I have been trying to determine the phosphate solubilization activity of several plant root nodule isolates. My goal is to measure the residual phosphate content in broth after 3 days incubation period. Is there any suitable colorimetric merhod for this purpose?
There are many methods for measuring phosphate. Many of them rely on the formation of a complex with molybdate under acidic conditions (often combined with Malachite Green dye), or the reaction of the enzyme nucleoside phosphorylase with a purine nucleoside. You can buy a commercially prepared reagents and kits from many vendors. Just type "phosphate assay kit" into your browser to see what is available, and choose one that us right for you based on cost, method of measurement (absorbance versus fluorescence), and sensitivity.
If you can use radioactivity, a very easy method would be to "spike" the broth with some 33P-labeled phosphate and measure the its depletion using liquid scintillation counting.
Just a warning about the Malachite Green methods. It is highly sensitive and not only to phosphate.
It detects even residual phosphate leaked from glassware, so you either have to rinse your glassware with HCl or use plasticware.
It may cross-react with various compounds in your reaction, especially if you'll use complex broth. When I was measuring activity of my protein, I had to simplify the buffer significantly and leave out everything unneccessary as betaine and BSA.
Is it somehow possible to measure only the remaining insoluble phosphate?
I have been trying to use a vanadate-molybdate reagent which yields a yellow coloured compound under acidic condition. The reagent works just fine in saline water, but it kind of caogulates organic components of the media, and this interferes with absorbance. Is there any way to overcome this problem?
If it's insoluble, you can simply centrifuge or filter it; wash it thoroughly and then analyze. The type of analysis depends on what everything is in the precipitate. If it's only phosphate (I suppose not), you could just weight it. Or you can perform elemental analysis. This is (somewhat basic) question to analytical chemist. So if you have some Department of analytical chemistry around, try to consult with them.
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