[1-13C] lactate was studied as hyperpolarized substrate to measure hepatic pyruvate carboxylase activity in vivo with fed and fasted conditions. Besides pyruvate, alanine and bicarbonate, 13C-aspartate could be detected from hyperpolarized lactate. Although the intracellular pyruvate pool size is tightly regulated, we found that lactate conversion to pyruvate and alanine is not saturated when we increased the concentration of hyperpolarized lactate from 30 to 60 mM, whereas the bicarbonate production was saturated. This study demonstrates the utility of hyperpolarized lactate to detect pyruvate carboxylase activity in vivo, and suggests that the metabolite ratio analysis should consider saturable enzyme activities.
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