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SynopsisThe binding of Mn2+ to double-stranded DNA in solution has been studied hy measuring the proton-relaxation enhancement of solvent water. The nmr experiments were carried out on DNAs of different base composition and showed the existence of weak and tight binding sites for Mn2+. The former sites are related to the electrostatic binding of the divalent cation with two contiguous phosphate groups, and the latter appear to vary almost proportionally with the number of G-C base pairs in the DNAs. Our results agree, in general, with the emerging evidence for a specific interaction of Mn2+ with the G C residues of DNA. Competition experiments with Mg2+ suggested that this binding of Mn2+ in DNA also involves the phosphate groups. More definitely, our data conform with the hypothesis made by Clement et al. of Mn2+ chelation between the phosphate and the nitrogen N7 of a deoxyguanyl unit of DNA. INTRODUCTIONIn the enzymatic synthesis of DNA, the Mn2+ cation fulfills the requirement for a metal activator, but behaves quite differently than Mg2+, which is considered to be a physiological cofactor.1,2 In elucidating such differences, it is important to gain more information on the specific interactions of these divalent cations with nucleotides.The binding of Mn2+ and Mg2+ with nucleosides, mononucleotides, polynucleotides, and RNA has been extensively in~estigated.~-~ The binding of these cations with DNA has been analyzed in detail by a variety of experimental m e t h~d s .~-l~The data presented for double-stranded DNA by a number of authors indicate that, in addition to the electrostatic binding of both cations with the phosphates, Mn2+ combines preferentially with the G-C residues of the nucleic acid.11-15 Clement et al.13 were the first to suggest that Mn2+ interacts specifically with DNA by forming a chelation complex with the N7 atom and the nearby phosphate of the A binding of Mn2+ with guanine in DNA duplexes would be hardly detectable by the. high-resolution nmr measurements mentioned above. However, this specific binding will be revealed by other types of nmr experiments, such as the study of the relaxation rates of solvent water protons in solutions of the paramagnetic Mn2+ ion and DNAs with different G-C contents. In fact, the water-proton-relaxation enhancement (PRE) due to the specific Mn-DNA interaction should definitely correlate with the proportion of guanine in the nucleic acids. In this paper we have analyzed the PRE induced in diluted aqueous solutions of Mn2+ salts by DNAs with various base compositions. Large differences in the PRE effects have been found.

The properties of proteoglycans (PGs) secreted into the growth medium by normal young and senescent human skin fibroblasts (HFs) were investigated. In both cases, the incorporation per cell of radioactive precursors into total PGs was similar. The polysaccharide chains of PGs from young and senescent HFs were mainly represented by galactosaminoglycuronans and showed a similar range of size distribution. However, galactosaminoglycuronans of PGs secreted by senescent HFs had a lower content of unsulphated disaccharides and a lower proportion of Dglucuronosyl residues. Moreover, senescent HFs released into the growth medium higher relative amounts of small PGs with chondroitin sulphate, dermatan sulphate chains, such as decorin.

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