Determination of Extinction Coefficients of Oat Phytochrome by Quantitative Amino Acid Analyses

Previously published extinction coefficients for phytochrome were based on indirect colori‐metric estimates of protein weight, mostly by the Lowry method, using bovine serum albumin as a standard. This paper reports revised values based on quantitative amino acid analyses of highly purified phytochrome. The molar extinction coefficient of the red‐absorbing form of phytochrome with 120000mol. wt subunits was found to be approximately 102000cm”1 per 120000moi. wt at 667nm. The molar extinction coefficients at 667 nm for phytochrome with 120000 mol. wt subunits and for the 60000 mol. wt chromopeptide of Pr were found to be approximately the same for samples of equivalent purity. Included in the amino acid analyses of phytochrome with 120000 mol. wt subunits are values for cysteine (11) and cystine (1), measured by two different techniques that gave the same results and agreed well with the estimate of total half‐cystine (14) obtained by an independent method. Tryptophan was quantitated by analysis of a p‐toluene sulfonic acid hydrolysate.