The stability of DNA in human cerebellar neurons.

Human tissues have carbon-isotope ratios (13C/12C) that reflect dietary ratios. This observation has been used to determine the extent of metabolic turnover of DNA in cells of the adult human cerebellum (90 percent of which are neuronal). If adult human neuronal DNA were metabolically stable, its 13C/12C would reflect that in the maternal diet during fetal development as nearly all neurons are formed during maturation of the fetal brain and do not undergo cell division thereafter. The 13C/12C ratios in the food chains and body tissues of Europeans differ from corresponding American ratios by about 50 parts per million on the average. Therefore, turnover was studied by comparing 13C/12C ratios in cerebellar DNA of American-born Americans, European-born Americans, and European-born Europeans. The 13C/12C ratios in cerebellar DNA from European-born Americans were closer to 13C/12C ratios in cerebellar DNA from European-born Europeans than from American-born Americans, indicating that there was little or no turnover of neuronal DNA.

Natural 13C/12C ratio variations in human populations.

Baseline data on the naturally occurring 13C/12C ratios of human tissue samples from several modern human populations are presented. Isotope ratios were determined for endogenous tissue samples (brain, liver and lung) from 20 routine unselected autopsy cases of residents of Long Island, New York. Comparable data were obtained from tissue samples from Bergen, Norway and Lucerne, Switzerland. Average values of 13C ratios reported in units of delta13C(0/00) relative to the PDB carbonate standard were -20.6 +/- 1.0 for 49 tissue samples from Switzerland, -20.7 +/- 1.0 for 21 tissue samples from Bergen, Norway and -16.2 +/- 1.0 for 89 tissue samples from Long Island, New York. These variations are discussed in terms of the isotopic carbon inventory in the respective diets of the populations sampled and also in terms of their implications for biomedical investigation.

The 13C/12C ratio in black pulmonary pigment: a mass spectrometric study.

Ratios of the two stable isotopes of carbon, 13C and 12C, were measured in black pulmonary pigment and in several endogenous tissues removed during 20 autopsies on Long Island, New York. The mass spectrometer used for carbon isotope ratio analyses has a precision of +/- 1 13C atom per million carbon atoms. The 13C/12C ratio was found to be distinctly lower in black pulmonary pigment than in endogenous tissues, the mean difference amounting to 65 13C atoms per million carbon atoms. The results demonstrate that the elemental carbon fraction of black pulmonary pigment is exogenous and constitutes roughly 30 per cent of the weight of the pigmented material. It was also observed that 13C/12C ratios in endogenous tissues from individual autopsy cases were not quite identical, tending to be slightly greater in corpus callosum than in liver or peripheral lung.

Variations in the isotopic composition of carbon in urban atmospheric carbon dioxide.

Measurement of the abundance of atmospheric carbon dioxide as an indicator of air pollution has been of very limited value because of variations in urban areas in the substantial concentration of natural carbon dioxide produced from combustion and noncombustion (natural) sources. A solution to this problem is the use of precise isotopic assay of ratios of carbon-13 to carbon-12 in atmospheric carbon dioxide. There is very little variation of carbon isotopic composition in samples taken over rural or urban areas where rapid mixing and diffusion of gaseous combustion products is possible. Significant differences in this composition in samples taken at centrally located points at street level in the lower Manhattan business district show an increase in concentration of atmospheric carbon dioxide of roughly 20 percent produced primarily by automobile exhaust.