A subset of xeroderma pigmentosum (XP) group E cells lack a factor that binds to DNA damaged by UV radiation. This factor can be purified to homogeneity as p125, a 125-kDa polypeptide. However, when cDNA encoding p125 is translated in vitro, only a small fraction binds to UV-damaged DNA, suggesting that a second factor is required for the activation of p125. We discovered that most hamster cell lines expressed inactive p125, which was activated in somatic cell hybrids containing human chromosome region 11p11.2-11cen. This region excluded p125 but included p48, which encodes a 48-kDa polypeptide known to copurify with p125 under some conditions. Expression of human p48 activated p125 binding in hamster cells and increased p125 binding in human cells. No such effects were observed from expression of p48 containing single amino acid substitutions from XP group E cells that lacked binding activity, demonstrating that the p48 gene is defective in those cells. Activation of p125 occurred by a "hit-and-run" mechanism, since the presence of p48 was not required for subsequent binding. Nevertheless, p48 was capable of forming a complex with p125 either bound to UVdamaged DNA or in free solution. It is notable that hamster cells fail to efficiently repair cyclobutane pyrimidine dimers in nontranscribed DNA and fail to express p48, which contains a WD motif with homology to proteins that reorganize chromatin. We propose that p48 plays a role in repairing lesions that would otherwise remain inaccessible in nontranscribed chromatin.

This study was designed to investigate the long-term metabolic adaptations and health effects of a low-carbohydrate, high-fat/protein diet in mice. One-month-old male ICR mice were fed a control, conventional high-carbohydrate diet (n=21) or an experimental low-carbohydrate, high-fat, high-protein diet (n=20). One pair of mice per group was euthanized at two-week intervals for five months for tissue analysis. Basic metabolic data, body and tissue weights, blood and plasma metabolite and lipid profiles, liver glycogen and protein content, and liver serine dehydratase and glucose-6-phosphate dehydrogenase activities were analyzed. The low-carbohydrate group gained significantly more weight (p<0.005 after 4 weeks) than the normally growing control group. Although ketosis was initially stimulated in the low-carbohydrate group, enzyme and tissue analysis suggest that gluconeogenic activity was sufficient to alleviate the effects of severe dietary carbohydrate restriction and allow for glucose metabolism close to that demonstrated in the control group.