FIGURE 5.

Mitochondrial PEPCK makes PEP in intact rat islets. A, schematic of PEP labeling strategy for islets. [1,2-¹³C]Glutamine (red circles and lines) is transported into the islet and deaminated to α-KG and then metabolized to succinyl-CoA loosing the label in carbon 1 to CO₂. Racemization across SCS and fumarase will evenly split the ¹³C label between carbons 1 and 4 of OAA. Thus, only half of the PEP produced by PEPCK-M will retain its label, whereas half is lost to CO₂. Unlabeled glutamine will produce malate and PEP that has only natural abundance carbon in all positions (white circles and blue lines). B, PEPCK-M activity in intact rat islets using [1,2-¹³C]glutamine. Rat islets were preincubated in DMEM base without glucose for 45 min and then with or without 5 mm [1,2-¹³C]glutamine for 45 min. The ¹³C enrichment of the M+1 isotopologue of malate M+1 and PEP are shown. Error bars are S.E. and significance by t test and ANOVA; *, p < 0.05; **, p < 0.01; ***, p < 0.001.