Preparation and characterization of two phosphopeptide fractions from calf thymus nuclei; similarity to corresponding fractions in the DNA prepared from the nuclei.

In this study two phosphopeptide (PP) fractions from purified, calf thymus nuclei, prepared under protective conditions were characterized. The nuclei were lysed and dialyzed, and the material in the dialysates was fractionated by anion exchange column chromatography. This revealed the presence of two main phosphopeptide (PP) fractions, numbers 1 and 5. Amino acid analysis of the fraction 1, occurring in the greatest amount, showed that, after deionization, it contained the metal ion complexes of phosphoserine, aspartic and glutamic acids in large amounts, and nine other amino acids. In the same way the two main PP fractions, P1 and P5, were obtained during the dialysis of the EDTA-reacted, highly purified DNA (N-DNA) prepared from calf thymus nuclei. Fraction 1 from nuclei had practically the same amino acid composition as fraction P1 from EDTA-reacted N-DNA. In both cases, the second main fraction obtained (fraction 5) had nearly the same amino acid composition as fraction 1, but the total Pser molar ratio, was about twice as high as for fraction 1. In contrast to the low molecular weights of 900-1400 daltons observed for PPs isolated directly from the dialysates in water without exposure to HCOOH, the high molecular weight of 10,260 daltons was obtained for the PP fraction 1, after exposure to HCOOH used in the gradient for the column chromatography.