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

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 α-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.