Geographic variations in the carbon isotope composition of the human diet and human hair were investigated. The carbon isotopic composition of common foodstuffs purchased in Chicago, USA, Tokyo, Japan and Munich, FRG, were determined by combustion and differential isotope ratio mass spectrometry. The dietary protein carbon for the United States (-18.1%) was calculated to be enriched in 13C relative to the Japanese (-21.2%) and the German (-23.6%) diets. To a large degree, the differences reflected the consumption of corn-fed animal products in the United States and Japan, as well as seafood in Japan. The carbon isotopic composition of hair (-16.4, -18.0 and -20.4%) for the three respective populations correlated with the calculated values of the dietary protein, but were 2-3% enriched in 13C. Changes in the isotopic composition of beard hair were shown to record the changes in dietary composition in travelers visiting the respective regions.
AblItraet. The influenceof different parameters on B 13 C-values in plants in vivo and on the 13C-isotope effect of the carboxylation reactions in vitro were examined using isotope ratio mass spectrometry.The primary fixation of carbon in photosynthesis occurs in so-called Cs-plants via the ribulosebisphosphate carboxylase (RuBP-C) reaction, and in Cs-plants via the phosphoenolpyruvate carboxylase (PEP-C) reaction. The observed difference in 813C-values" of the organic material of these two groups of plants is a consequence of the different isotope discrimination by the two carboxylation reactions. I Changes of ecological conditions during growth are accompanied by variations of the 8 13 C-values within these groups or even for a single plant species.r"In particular, rise of temperature during growth generally leads to more negative 8 I'C-values; therefore the temperature coefficient of the discrimination effect seems to be positive. On the other hand, from corresponding in vitro experiments on isotope discrimination by the carboxylation reactions of RuBP-C and PEP-C, rather contradictory results were reported.v'" Whelan et aI. 1 observed a strong decrease in isotope discrimination by RuBP-C from sorghum with increasing temperature, whereas Christeller et al.6 found temperature independence for the isotope effect of the RuBP-C-reaction (enzyme from soy beans). As methods and enzymes used in these experiments differed from each other, and as from general experience in enzyme kinetics a negative temperature coefficient of the reaction was to be expected, we investigated the influence of different parameters on 8
13C-values of plants in vivo and, in parallel experiments, on the I'C-isotope effect of the carboxylation reactions in vitro.
EXPERIMENTALSamples of plant material (5 mg) were converted to CO 2 in a commercial micro-combustion apparatus in a He/02 gas flow. The CO2 was isolated in a liquid nitrogen trap and then expanded into the sample reservoir of an isotope ratio mass spectrometer with double inlet system (MM 903, VG Micromass). The B 13 C-values were determined relative to a suitable standard. The in vitro experiments with enzymes were performed on incubation mixtures with an excessof carbon dioxide. After defined partial reaction, controlled spectrophotometrically, the remaining CO 2 was transferred by a He carrier gas flowinto a liquid nitrogen trap and assayed for 13C as mentioned above; the kinetic isotope effect was determined from the change of the B13C-value of the CO 2 before and after incubation.
RESULTS AND DISCUSSIONThe isotope effect of the PEP-C-reaction was klJk l, = 1.002 ± 0.0003 (25°C; pH 8.0) with a temperature coefficient of -0.OOOO3/"C; no difference was found between enzymes from wheat and maize, representing C,-and Cz-plants respectively. The low value observed for k I2/k 13 is in line with the synchronous mechanism for the carboxylation step proposed by Utter.' The dependence of the kinetic isotope effect of the PEP-C-reaction on the pH-value could be mainly correlated with the th...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.