Amino acid-activating systems from pig liver.

1. The polarization spectrum of insulin, ribonuclease and zein is qualitatively identical with that of tyrosine or cresol, but the lower absolute values of the principal polarization indicate the existence of energy transfer among the tyrosine residues. 2. The polarization spectrum of proteins containing tryptophan is similar to that of N-glycyltryptophan but differs from it in the lower values of the polarization in the 270 m, region. The ratio of the polarization on excitation by 305 m, and by 270 m, (P305/P270 ratio), which varies from 14 to 1X7 in the simple indole derivatives, is found to be greater than two in nine out of ten globular proteins studied. 3. The polarization spectrum in 50% propylene glycol-water at 700 shows an increased P305/P270 ratio (2.3-3.0), and the polarization spectrum in 8M-urea shows this ratio decreased to the range 1'4-1 9 in eight out of ten proteins studied. 4. The changes of proteins in urea are only partially reversible. The soluble fractions obtained in three proteins had a distinctly changed polarization spectrum with a lowered P305/P270 ratio. 5. Though energy transfer among the tryptophan residues may play a part in the observed effects, it is believed that a change in the relative intensities of the G-S1 and G-S2 transitions in tryptophan can explain these effects equally well.