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Cardiac holotroponin can be phosphorylated at serine 23 and/or 24 in the heart-specific region of bovine troponin I. When isolated freshly it is composed of a mixture of non-, two mono-, and bisphosphorylated species. At neutral pH the monophosphorylated form carrying phosphate at serine 24 yields a resonance signal at 4.6 ppm and that carrying phosphate at serine 23 at 4.4 ppm; the two phosphate groups of the bisphosphorylated form yield only one 31P-NMR signal at 4.2 ppm. From the chemical shift dependence on pH, pKa values have been estimated to be 5.3 and 5.6 for the phosphate groups at serine 24 and serine 23, respectively. Both phosphates of the bisphosphorylated form exhibit very similar pKa values of approximately 5.8. Separation of bisphosphotroponin I from the complex results in a downfield shift and the appearance of two 31P-NMR signals at positions comparable to those of the two monophospho forms. Complex formation of cardiac troponin I with C or T does not alter the spectrum obtained with isolated troponin I; however, the original troponin spectrum is restored by reconstitution of the holocomplex from all three components T, I, and C. Two signals are also observed with a bisphosphorylated synthetic peptide [PVRRRS(P)S(P)ANYR] representing the phosphorylation domain. pKa values of about 5.3 and 5.6 have been determined for serine 7 (corresponding to serine 24 of troponin I) and serine 6 of the peptide (corresponding to serine 23 of troponin I).

The rate constant for free fatty acid influx (kl) was studied in normal and ischemic myocardium.In 15 normal subjects and 30 patients with coronary artery disease, 20MTl and 15-(p-_231-iodophenyl)-pentadecanoic acid (IPPA) were administered during exercise under fasting conditions and at rest. In 10 patients, the study was repeated after percutaneous transluminal coronary angioplasty; in three patients, the study was repeated after infarction. The initial accumulation of IPPA, related to that of 2`1TI (both background and crossover corrected), was used for determinations of the regional rate constant of IPPA influx into myocardial tissue (kl*). In normal subjects, no significant differences in kl* between major myocardial segments were found; the average value of kl* was 0.57 0. 13/min (mean + SD) at rest and 0.42 0.06/min at exercise (average workload, 123 47 W). With increasing free fatty acid plasma concentration and perfusion, free fatty acid influx increased in a saturable fashion. The Michaelis-Menten constant (KM*) and the maximal velocity (Vm.*) for IPPA influx into myocardial tissue were estimated to be 470 nmol/g and 430 nmol/g * min, respectively. In ischemic areas, k1* was reduced to 57±18 % of k1* value in nonaffected segments. The areas were larger than those showing reduced 201T1 uptake. Preinfarction and postinfarction studies showed that the size of 20`TI defects in postinfarction images corresponded with the size of the area with reduced kl* observed in preinfarction scintigrams. Revascularization led to an increase of 20`TI uptake and to normalization of kl*.

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Fatty acid binding protein has been found in rat aortic endothelial cell membrane. It has been identified to be a