Pseudoketogenesis in the perfused rat heart.

Area of Science:

• Biochemistry
• Metabolic Research
• Cardiovascular Physiology

Background:

• Ketogenesis, the production of ketone bodies, is typically quantified using tracer dilution methods for (3R)-hydroxybutyrate and acetoacetate.
• Existing methods may not account for isotopic exchange processes that can occur independently of net ketogenesis.
• Understanding these processes is crucial for accurate metabolic flux analysis in cardiac tissue.

Purpose of the Study:

• To investigate the phenomenon of isotopic exchange in tracer measurements of ketogenesis.
• To identify and characterize 'pseudoketogenesis' in perfused working rat hearts.
• To determine the contribution of isotopic exchange to the dilution of ketone body tracers.

Main Methods:

• Perfusing working rat hearts with buffers containing labeled acetoacetate ([4-3H]- plus [3-14C]acetoacetate).
• Perfusing hearts with unlabeled acetoacetate and labeled palmitate ([1-14C]-, [13-14C]-, or [15-14C]palmitate).
• Measuring specific activities of (3R)-hydroxybutyrate, acetoacetate, and acetyl-CoA to quantify isotopic exchange.

Main Results:

• Observed dilution of tracer specific activities for (3R)-hydroxybutyrate and acetoacetate in the absence of net ketogenesis, termed pseudoketogenesis.
• Demonstrated isotopic exchange via reversibility of mitochondrial 3-oxoacid-CoA transferase and acetoacetyl-CoA thiolase reactions.
• Calculated minimal pseudoketogenesis rates ranging from 19% to 32% of net ketone uptake.

Conclusions:

• Isotopic exchanges significantly impact the accuracy of in vivo ketogenesis measurements using tracer dilution.
• Pseudoketogenesis, driven by reversible enzymatic reactions, leads to underestimation of net ketogenesis.
• These findings necessitate re-evaluation of standard methods for measuring ketone body metabolism in the heart.