Jacobson, Bruce S. and Smith, Bruce N. and Epstein, Samuel and Laties, George G. (1970) The Prevalence of Carbon-13 in Respiratory Carbon Dioxide As an Indicator of the Type of Endogenous Substrate. The change from lipid to carbohydrate during the respiratory rise in potato slices. Journal of General Physiology, 55 (1). pp. 1-17. ISSN 0022-1295 http://resolver.caltech.edu/CaltechAUTHORS:JACjgp70
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Isotope discrimination is a common feature of biosynthesis in nature, with the result that different classes of carbon compounds frequently display different 13C/12C ratios. The 13C/12C ratio of lipid in potato tuber tissue is considerably lower than that for starch or protein. We have collected respiratory CO2 from potato discs in successive periods through 24 hr from the time of cutting—an interval in which the respiration rate rises 3–5-fold. The 13C/12C ratio of the evolved CO2 was determined for each period, and compared with the 13C/12C ratios of the major tissue metabolites. In the first hours the carbon isotope ratio of the CO2 matches that of lipid. With time, the ratio approaches that typical of starch or protein. An estimation has been made of the contribution of lipid and carbohydrate to the total respiration at each juncture. In connection with additional observations, it was deduced that the basal, or initial, respiration represents lipid metabolism —- possibly the alpha-oxidation of long chain fatty acids -— while the developed repiration represents conventional tricarboxylic acid cycle oxidation of the products of carbohydrate glycolysis. The true isotopic composition of the respiratory CO2 may be obscured by fractionation attending the refixation of CO2 during respiration, and by CO2 arising from dissolved CO2 and bicarbonate preexisting in the tuber. Means are described for coping with both pitfalls.
|Additional Information:||Copyright © 1970 by The Rockefeller University Press. RUP grants the public the non-exclusive right to copy, distribute, or display the Work under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 Unported license, as described at http://creativecommons.org/licenses/by-nc-sa/3.0/ and http://creativecommons.org/licenses/by-nc-sa/3.0/legalcode. Received for publication 16 July 1969. This work was generously supported by a grant from the United States Public Health Service to Dr. Laties, and by National Science Foundation Grant BG 7517 to Dr. Epstein. Mr. Jacobson is indebted to the United States Public Health Service for National Institute of Health Fellowship 5-F1-GM-33,256 from the National Institute of General Medical Sciences. This is contribution 1650 from the Division of Geological Sciences, California Institute of Technology.|
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