Use the Lowry et al (1951) method for protein estimation. It is highly reliable

First you should homogenize the tissue. I put zirconium oxide beads together with the tissue in a tube in a homogenizer. Be sure to put enough buffer in the tubes or the beads will break it! We succesfully use a salt buffer (0.4M NaCl, 10mM NaPO4) with 0.05% Tween20 and a protease inhibitor cocktail to keep your protein intact in the buffer afterwards (premade tablets or you can add components like PMSF, EDTA, ... separately) (published by Coelho et al, Arthritis & Rheumatism 2008:58(8)).

If you would use a buffer containing Tween20, you can use the Lowry or similar BCA assay for protein estimation, since they are not hindered by detergents. Also in  your particular case they are a good choice because denaturating factors like ureum (very high in liver samples) are not influencing the measurment.

Thank you Adekunle and rik