Incorporation of Acetate Carbon Into Rat Liver Glycogen by Pathways Other Than Carbon Dioxide Fixation

This communicationpresents direct evidence that the intact mammal can incorporate acetate carbon into liver glycogen by pathways other than CO2 fixation.It has been reported that Cl3 entering rat liver glycogen by CO2 fixation could be detected only in positions 3 and 4 of the glucose molecule,l and that the Cl3 of carboxyl-labeled acetate, likewise, could be detected only in these positions.2Subsequent experiments have shown that these are also the only positions in which excess isotope appears in rat liver glycogen after feeding carboxyl-labeled propionate and butyrate.3By the position of the labeled carbon, therefore, it is impossible to exclude CO2 fixation as the pathway of conversion of these fatty acids to glycogen.Buchanan et ~1.~ from similar experiments, using acetate, propionate, and butyrate labeled with C" in the carboxyl groups, inferred that acetate carbon enters liver glycogen only via CO2 fixation, but that an additional mechanism of conversion is involved in the cases of propionate and butyrate.Our own results with the carboxyl-labeled acids indicated, when allowance was made for CO2 fixation, that not only the propionate and butyrate enter glycogen in a form other than COZ, but also the acet'at'e.The findings of Rittcnbcrg and Bloch" in feeding experiments lvith carboxyl-labeled