Noninvasive probing of citric acid cycle intermediates in primate liver with phenylacetylglutamine

In human and primate liver, phenylacetate and glutamine form phenylacetylglutamine, which is excreted in urine. Probing noninvasively the labeling pattern of liver citric acid cycle intermediates with phenylacetylglutamine assumes that the labeling pattern of its glutamine moiety reflects that of liver α-ketoglutarate. To validate this probe, we infused monkeys with [U-¹³C₃]lactate, [3-¹³C]lactate, [1, 2- ¹³C₂]acetate, [2-¹³C]acetate, [U-¹³C₃]glycerol, or 2-[3- ¹³C]ketoisocaproate and compared the labeling patterns of urinary phenylacetylglutamine with those of glutamate and glutamine in liver, plasma, muscle, and kidney and liver α-ketoglutarate. Only with [U-¹³C₃]lactate or [3-¹³C]lactate does the labeling pattern of phenylacetylglutamine reflect patterns of liver α-ketoglutarate and glutamate. With [¹³C]acetate, muscle and kidney glutamate are more labeled than liver metabolites. This confirms that with [¹³C]acetate, the labeling pattern of liver metabolites is influenced by ¹³CO₂ and [¹³C]glutamine made in peripheral tissues. Our data validate the use of phenylacetylglutamine labeled from [3-¹³C]lactate or [3-¹³C] pyruvate to probe noninvasively the pyruvate carboxylase-to-pyruvate dehydrogenase flux ratio in human subjects.