Assessment of hepatic pyruvate carboxylase activity using hyperpolarized [1-¹³ C]-l-lactate

To evaluate the utility of hyperpolarized [1-¹³ C]-l-lactate to detect hepatic pyruvate carboxylase activity in vivo under fed and fasted conditions.

[1-¹³ C]-labeled sodium L-lactate was polarized using a dynamic nuclear polarizer. Polarization level and the T₁ were measured in vitro in a 3 Telsa MR scanner. Two groups of healthy rats (fasted vs. fed) were prepared for in vivo studies. Each rat was anesthetized and intravenously injected with 60-mM hyperpolarized [1-¹³ C]-l-lactate, immediately followed by dynamic acquisition of ¹³ C (carbon-13) MR spectra from the liver at 3 Tesla. The dosage-dependence of the ¹³ C-products was also investigated by performing another injection of an equal volume of 30-mM hyperpolarized [1-¹³ C]-l-lactate.

T₁ and liquid polarization level of [1-¹³ C]-l-lactate were estimated as 67.8 s and 40.0%, respectively. [1-¹³ C]pyruvate and [1-¹³ C]alanine, [¹³ C]bicarbonate ( $$ ) and [1-¹³ C]aspartate were produced from hyperpolarized [1-¹³ C]-l-lactate in rat liver. Smaller $$ and larger aspartate were measured in the fed group compared to the fasted group. Pyruvate and alanine production were increased in proportion to the lactate concentration, whereas the amount of $$ and aspartate production was consistent between 30-mM and 60-mM lactate injections.

This study demonstrates that a unique biomarker of pyruvate carboxylase flux, the appearance of [1-¹³ C]aspartate from [1-¹³ C]-l-lactate, is sensitive to nutritional state and may be monitored in vivo at 3 Tesla. Because [¹³ C] $$ is largely produced by pyruvate dehydrogenase flux, these results suggest that the ratio of [1-¹³ C]aspartate and [¹³ C] $$ (aspartate/ $$ ) reflects the saturable pyruvate carboxylase/pyruvate dehydrogenase enzyme activities.