Two DNA polymerases purified from normal human lymphocytes are distinguishable from the viral reverse transcriptases of avian myeloblastosis virus and Mason-Pfizer monkey virus by their relative affinity for select templates. In this respect, the activity of the two normal human lymphocyte polymerases closely resembles the activity of Escherichia coli DNA polymerase 1. The viral and cellular DNA polymerases are equally active with the nonspecific template, poly(rA) . poly(dT). Criteria for distinguishing the activity of viral reverse transcriptase are discussed.

DNA polymerase III is an enzyme activity in eukaryotic cells which under certain conditions shows strong preference for polyadenylic acid as template when primed by oligodeoxythymidylate. Its first complete separation from other DNA polymerases in human lymphoblasts is reported. This enzyme is biochemically and immunologically distinct from DNA polymerase I and from viral reverse transcriptase from a primtate type C virus.

The DNA polymerase from the Mason-Pfizer monkey virus (M-PMV), an RNA tumor virus not typical type-C or type-B, has been purified a thousand-fold over the original crude viral suspension. This purified enzyme is compared to a similarly purified DNA polymerase from the primate woolly monkey virus, a type-C virus. The two enzymes have similar template specificities but differ in their requirements for optimum activity. Both DNA polymerases have a pH optimum of 7.3 in Tris buffer. M-PMV enzyme has maximum activity with 5 mM Mg2+ and 40 mM potassium chloride, whereas the woolly monkey virus optima are 100 mM potassium chloride with 0.8 mM Mn2+. The apparent molecular weight of the M-PMV enzyme is approximately 110,000, whereas the woolly monkey virus polymerase is approximately 70,000. The biochemical properties of these two enzymes were also compared to a similarly purified enzyme from a type-C virus from a lower mammal (Rauscher murine leukemia virus). The results show that more similarity exists between the DNA polymerases from viruses of the same type (type-C), than between the polymerases from viruses of different types but from closely related species.

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The kinetic effect of added common anion in the solvolysis of dimethyl-t-butyl s~~lphonium halides in ethanol-water mixtures is analyzed in terms of a mechanism involving solvolysis through a n ion-pair intermediate. The simple ion-pair role, however, does not account for the kinetic behavior a t higher added common-anion concentrations. A normal salt effect and further involvement of the ion pair in Sx2 attacl; by common anion are compared as possible explanations for the observed kinetic behavior. INTRODUCTIOXEvidence presented in the previous paper of tliis series ( I ) was interpreted as indicative of the increasing role of an ion-pair niechanism in the solvolysis of dimethyl-t-butyl sulphonium salts as the dielectric constant of the mediuni is reduced. Further experimental evidence in support of this proposed nlechanism is presented in this paper. I t is clear from equation [ I ] tliat such a mechanism should be affected by the concentration of coiiinion anion X-. Higher concentrations should favor ion-pair forniation and a higher proportion of the over-all solvolysis should proceed through the ion-pair intermediate. The over-all rate of solvolysis (kobs) of dimethyl-t-butyl sulphoiiiuin iodide and chloride has therefore been measured as a functioi~ of both added coninion anion and solvent coniposition.I11 the previous paper of tliis series it was silo\\-ii tliat if the ion-pair niecl~anism is correct, k , , > k+. Consequently, added common anion should increase the over-all rate of solvolysis (kobs) since a greater proportion of tlie solvolytic process will proceed through the faster ion-pair pathway. As the dielectric constant of the solvolytic mecli~~iii is reduced, tlie ion-pair association constant ( K , ) will increase and together witli increasing concentration of common anion these factors \\-ill result in virtually complete conversion of the sulphonium ion to the ion-pair state. The over-all rate of solvolysis slioulcl therefore increase witli added common anion and lower dielectric constant but should approach an upper limiting value as the degree of ion pairing approaches 100%. EXPERIMEXTAL METHOD .AND RESULTSAll rates were measured by tlie radiochemical flow counting technique of Hyne and Wolfgang (2). C14-labelled dimethyl-t-butyl sulphonium iodide was prepared as described previously ( 2 ) and converted to the chloride by metathesis witli freshly prepared silver chloride. Solvent mixtures were prepared by weight from doubly distilled water (lo-' mhos) and redistilled absolute ethanol. Reagent grades of lithium chloride and iodide 'Manuscript