Trapping and metabolism of radioiodinated PHIPA 3-10 in the rat myocardium.
PHIPA 3-10 [13-(4'-iodophenyl)-3-(p-phenylene)tridecanoic acid] is a p-phenylene-bridged, radioiodinated omega-phenyl fatty acid that has recently been developed to study coronary artery disease or cardiomyopathies. Here, we demonstrate that PHIPA 3-10 exhibits the characteristics of a long-chain fatty acid, including its ability to be efficiently taken up by myocytes and to function as a substrate for beta-oxidation before it is trapped.


METHODS
Myocardial metabolism of carrier-added and carrier-free 131I-PHIPA 3-10 preparations were investigated in rats in vivo and in isolated Langendorff rat hearts. Heart extracts were analyzed by high-performance liquid chromatography, negative-ion electrospray mass spectrometry and investigation of intracellular distribution using density-gradient centrifugation.


RESULTS
A single, rapidly formed metabolite was found in the heart extract and also, surprisingly, in the hydrolyzed lipids. The total amount of metabolite increased from 43% to 51% between 15 and 60 min postinjection. By high-performance liquid chromatography comparison with synthetic potential catabolites, the metabolite was assigned the name PHIPA 1-10 [11-(4'-iodophenyl)-1-(p-phenylene)undecanoic acid] and was the product of one beta-oxidation cycle. Additional proof was obtained from the mass spectrometric analysis of the metabolite formed in vivo. The formation of this metabolite could be suppressed by Etomoxir, a carnitine palmitoyl transferase I inhibitor, indicating beta-oxidation of 131I-PHIPA 3-10 in mitochondria. Final evidence for the involvement of mitochondria in the degradation of 131I-PHIPA 3-10 was obtained by density-gradient centrifugation of homogenized rat heart tissue. The position of the labeled free PHIPA 3-10 and free metabolite peaked within the fraction containing mainly mitochondria.


CONCLUSIONS
In spite of its bulky structure, 131I-PHIPA 3-10 is extracted by the myocardium in a manner similar to the extraction of the unmodified fatty acid analog, IPPA. The retention of PHIPA 3-10 in heart muscle results from the presence of the p-phenylene group, which prevents more than one beta-oxidation cycle. Intracellular free PHIPA 3-10 and free PHIPA 1-10 are present in the mitochondria, whereas most of the esterified metabolite was found in the cytosolic lipid pool. Hence, the rapid appearance of PHIPA 1-10 in the lipid pool must be accounted for by mitochondrial leakage or by an unknown in-out transport system.
Eisenhut
M
M
Department of Nuclear Medicine, and Institute for Biochemistry I, University of Heidelberg, Germany.
Lehmann
W D
WD
Hull
W E
WE
Just
W W
WW
Hoffend
J
J
Zehelein
J
J
Zimmermann
R
R
D000818:Animals; D003287:Contrast Media / Q000737:chemistry / Q000493:pharmacokinetics; D006321:Heart / Q000000981:diagnostic imaging*; D007457:Iodine Radioisotopes / Q000493:pharmacokinetics; D008297:Male; D009206:Myocardium / Q000378:metabolism*; D010666:Phenylpropionates / Q000737:chemistry / Q000493:pharmacokinetics; D011877:Radionuclide Imaging; D051381:Rats; D017207:Rats, Sprague-Dawley
C102993:4-(10-(4-iodophenyl)decyl)benzenepropanoic acid; D003287:Contrast Media; D007457:Iodine Radioisotopes; D010666:Phenylpropionates