Specificity and Interfering Substances-Few substances encountered in
biological work cause serious interference. Only a little color was obtained with either acid extracts or the lipides extracted from five different
tissues (Table III). Consequently measurements on non-extracted whole
tissue would be in error by only 3 t,o 6 per cent, whereas values based on N
determination would be overestimated by 15 to 20 per cent.
Uric acid (16), guanine, and xanthine (21, 22) react with the Folin reagent. Guanine gives about 50 per cent more color than serum protein,
weight for weight. The color is not enhanced by copper. Curiously, guanosine does not react appreciably. Hypoxanthine gives no color if purified (21). No more than a trace of color was obtained with adenine, ade-
by guest on March 17, 2019 http://www.jbc.org/ Downloaded from
LOWRY, ROSEBROUGII, FARR, AND RANDALL 273
nosine, cytosine, cytidine, uracil, thymine, or thymidine (see also Funk
and Macallum (22)).
Neither color nor interference with protein color development was observed with the following substances at the given &al concentrations:
urea (0.5 per cent), guanidine (0.5 per cent), sodium tungstate (0.5 per
cent), sodium sulfate (1 per cent), sodium nitrate (1 per cent), perchloric
acid (0.5 per cent neutralized), trichloroacetic acid (0.5 per cent neutralized), ethyl alcohol (5 per cent), ether (5 per cent), acetone (0.5 per cent),
zinc sulfate (0.1 per cent), barium hydroxide (0.1 per cent).
Most phenols, except nitrophenols, reduce the reagent (16) ; t,herefore
thymol and to a lesser degree sulfosalicylic acid interfere,