Metabolism of 15-(4'-[123I]iodophenyl)pentadecanoic acid ([123I]IPPA) in the rat heart; identification of new metabolites by high pressure liquid chromatography and fast atom bombardment-mass spectrometry.

The metabolism of 15-(4'-iodophenyl)pentadecanoic acid (IPPA) in the heart muscle is commonly believed to end at 4-iodobenzoic acid as the main and final product of beta-oxidation. However, investigation of the metabolic fate of IPPA in Langendorff rat hearts using high pressure liquid chromatography (HPLC) and negative fast atom bombardment mass spectrometry (FAB-MS) revealed new results. After perfusing isolated rat hearts with [123I]IPPA, metabolites were monitored by HPLC using simultaneous detection of gamma-radioactivity and u.v. absorbance. The identification of HPLC separated metabolites was based on their nominal molecular weights as determined by negative FAB-MS. According to these measurements five catabolites were identified with decreasing concentration: 3-(4'-iodophenyl)propanoic acid >> 3-(4'-iodophenyl)propanoic acid = 5-(4'-iodophenyl)-3-hydroxypentanoic acid >> 4-iodobenzoic acid. Additionally, an anabolic metabolite was found exclusively in the lipid ester fraction. From the hydrolysed heart lipids this compound was identified as 11-(4'-iodophenyl)undecanoic acid. Its formation is explained by the action of cytosolic fatty acid synthetase on IPPA catabolites. This metabolic behaviour may be of importance for the interpretation of sequential heart scintigraphy performed with [123I]IPPA.