13C labeling patterns in products of glycolysis, PPP and mitochondrial Krebs cycle with [U-13C]-Glc as tracer. The Krebs' cycle reactions are depicted without (panel A) or with (panel B) anaplerotic pyruvate carboxylation (PC) and the labeling patterns illustrated represent one turn of cycle activity. Two distinct 13C labeling patterns in Glu result from the cycle reactions with (panel B) or without PC (panel A), i.e. 13C labeling at C2,3 or C4,5, respectively. Isotopic scrambling occurs at the symmetric succinate, leading to the redistribution of 13C labels into C1 and C2 or C3 and C4 (panel A). Also depicted in panel B is the malate/Asp shuttle across the mitochondrial membrane, where the export of [13C-1,2,3]-Asp into the cytosol leads to the synthesis of [13C-1,2,3]-malate. Red C and green C represent 13C labeled carbons; solid and dashed arrows denote single and multi-step reactions, respectively; open arrows in panel A delineate unlabeled starting oxaloacetate (OAA) from 13C-labeled OAA after one turn; open arrow in panel B denotes the start of a second cycle; double headed arrows indicate reversible or exchange reactions; SCS denotes succinyl CoA synthetase.