Pseudoketogenesis in the perfused rat heart.

Ketogenesis is usually measured in vivo by dilution of tracers of (3R)-hydroxybutyrate or acetoacetate. We show that, in perfused working rat hearts, the specific activities of (3R)-hydroxybutyrate and acetoacetate are diluted by isotopic exchanges in the absence of net ketogenesis. We call this process pseudoketogenesis. When hearts are perfused with buffer containing 2.3 mM of [4-3H]- plus [3-14C]acetoacetate, the specific activities of [4-3H] and [3-14C]acetoacetate decrease while C-1 of acetoacetate becomes progressively labeled with 14C. This is explained by the reversibility of reactions catalyzed by mitochondrial 3-oxoacid-CoA transferase and acetoacetyl-CoA thiolase. After activation of labeled acetoacetate, the specific activity of acetoacetyl-CoA is diluted by unlabeled acetoacetyl-CoA derived from endogenous fatty acids or glucose. Acetoacetyl-CoA thiolase partially exchanges 14C between C-1 and C-3 of acetoacetyl-CoA. Finally, 3-oxoacid-CoA transferase liberates weakly labeled acetoacetate which dilutes the specific activity of extracellular acetoacetate. An isotopic exchange in the reverse direction is observed when hearts are perfused with unlabeled acetoacetate plus [1-14C]-, [13-14C]-, or [15-14C]palmitate; here also, acetoacetate becomes labeled on C-1 and C-3. Computations of specific activities of (3R)-hydroxybutyrate, acetoacetate, and acetyl-CoA yield minimal rates of pseudoketogenesis ranging from 19 to 32% of the net uptake of (3R)-hydroxybutyrate plus acetoacetate by the heart.