Tris is a pH buffer, if the pH of the base is adjusted with an acid, usually HCl. For microsomes, you would probably want to use pH 7.4. It is important to maintain a constant pH in the physiological range, which is the purpose of the buffer.

EDTA is a chelator of divalent and trivalent cations that might be present as contaminants and interfere in the assay. The assay buffer probably also will contain a Mg2+ salt, most likely MgCl2. EDTA will chelate Mg2+, so the Mg2+ concentration must be above the EDTA concentration. Since the affinity of EDTA for other cations is much higher than its affinity for Mg2+, it will still bind up these other cations in the presence of Mg2+. The pH of the EDTA stock solution should be neutralized with NaOH to a physiological pH (it is usually supplied at pH 8).

Thank you for the answer Adam B Shapiro ! Is there a reason why Tris is commonly used over other buffers for microsomes though? I am concerned as Tris increases in pH when cold, of which I plan to spin my microsomes at 4C, so adjusting Tris to pH 7.4 at room temp will result in a higher pH during preparation.

Janice Goh , all buffers change pH with temperature variation. Some more than others. pH limits your buffer selection. The available pH for a buffer is within the range of pKa ± 1. The pKa of MES is 6.1 at room temperature so you can't use it if you want the pH to be buffered near 7.4. Phosphate buffers with pKa = 7.2 so you could use it with respect to pH unless you are studying DNA binding enzymes because how does an enzyme know the difference from a phosphodiester backbone of DNA and phosphate?

Anyway, I don't think Tris will give you much trouble but if it does Hepes is also a good buffer at pH 7.4 and the pH drifts less with temperature.