METHOD FOR ASSAYING COENZYME A MOLECULES IN BIOLOGICAL'SAMPLES FIELD OF THE INVENTION The present invention relates to a method for quantitative assay of Coenzyrae A (hereinafter referred to as "CoA") molecules in biological samples. BACKGROUND ART CoA molecules are important class of compounds indispensable for maintaining life functions, being involved in the paths of biosynthesis, decomposition and conversion of fatty acids, hormone synthesis and regulation, the TCA cycle, etc. Methods for accurately assaying CoA molecules concentrations in biological samples have been a target of research, with emphasis on the role of such concentrations as markers for CoA molecules in functional analysis research on lipid metabolism. One of the CoA molecules, malonyl CoA, is a constituent of the lipid metabolism mechanism involved in fatty acid oxidation in the mitochondria and lipid synthesis, and therefore performs a crucial role for regulation of lipid metabolism; it has received much attention as a regulatory factor essential for regulating lipid energy metabolism in the heart and skeletal muscle and expression of neuropeptide Y in the cerebral hypothalamus, as well as for controlling dietary intake and energy consumption. Various methods for assaying CoA molecules have been developed in the past, including enzyme methods and HPLC. Examples of enzymatic assay methods include the method of Guynn et al. (Guynn, R.W., Methods Enzymol., 1975, 35, 312) and the method of McGarry et al. (McGarry, J.D., J. Biol. Chem., 1978, 253, 22, 8291). For example, when the assay target is malonyl CoA, malonyl CoA-dependent uptake of adiolabeled acetyl CoA or the like is measured. However, this method has limited applicability because its assay target is a specific CoA and adjustment must be made due to the copresence of other CoA molecules during the assay, leading to inaccurate measured values in some cases, while the procedure is also complicated. Numerous reports have been published on assay of CoA molecules by UV-HPLC (see, for example, Hosokawa, Y., Anal. Biochem., 1978, 91, I, 370, Demoz, A., J. Chromatogr., B: Biomed. Appl., 1995, 667, 1, 148). CoA concentrations in muscle, cardiac and hepatic biological samples are assayed by this method. However, due to the low sensitivity of UV-HPLC, it has poor reproducibility for highly contaminated samples such as those from organs, with brain concentrations being particularly difficult to measure, and therefore a method offering higher sensitivity and reproducibility has been desired. LC-MS has been developed as an alternative method for achieving higher sensitivity (see Buchholz, A., Anal. Biochem., 2001, 295, 129). This method accomplishes assay of acetyl CoA concentrations in cells using a three-dimensional ion trap mass spectrometer. Although success has been achieved with this method for detection and peak separation of a particular CoA as the assay target, the accuracy and reproducibility are insufficient because the method is an absolute calibration curve method and does not use an internal standard substance. Still, the assay target is acetyl CoA which has low polarity and is easily separated by HPLC. Its additional advantages are that cellular samples have low amounts of contaminants that can interfere with measurement, and that acetyl CoA concentrations are high in samples. The method cannot be adequately applied, however, for assay of CoA molecules in cases of highly polar forms whose separation is relatively difficult by HPLC, in cases of samples such as animal organs with high amounts of contaminants that can interfere with nieasurement, and in cases of low concentrations in samples (such as malonyl CoA in animal organs). The present invention solves the problems described above by providing a concentration assay method with excellent sensitivity and reproducibility for CoA molecules in biological samples. DISCLOSURE OF THE INVENTION As a result of much diligent research on the subject described above, the present inventors have discovered the following method. Specifically, the invention provides a method for assaying concentrations of Coenzyme A molecules in biological samples, the method being characterized by comprising a step of extraction from a biological sample using a strongly acidic solution, a step of solid phase extraction, a step of adding an internal standard substance, and a step of detection by LC-MS. The invention further provides the aforementioned method for assaying Coenzyme A molecules characterized in that the step of extracting the Coenzyme A molecule from the biological sample is a step wherein a freeze-shattered biological sample is agitated in a perchloric acid solution and the supernatant is subjected to centrifugal separation. The solid phase extraction step is characterized by being a step wherein the supernatant obtained by extraction of the Coenzyme A with a strongly acidic solution is neutralized, and then applied to a reverse phase cartridge packed with silica gel containing an octadecylsilyl group or octylsilyl group, washed with an aqueous solvent, and eluted with an organic solvent. In particular, the supernatant is applied after conditioning the reverse phase cartridge with acetonitrile and 1 M ammonium acetate solution, and elution is performed with an acetonitrile and ammonium acetate mixture. The invention still further provides the aforementioned method for assaying Coenzyme A molecules characterized in that 'the Coenzyme A molecule is a fatty acid Coenzyme A ester, and the internal standard substance is a structural analog of the Coenzyme A molecule. The invention still further provides the aforementioned method for assaying Coenzyme A molecules characterized in that the fatty acid Coenzyme A ester is a Coenzyme A ester of a short chain fatty acid with 2-8 carbons in the main carbon chain, the structural analog has a difference of no more than 3 carbons with the Coenzyme A molecule and has at least 3 of the hydrogens of the main chain substituted with deuterium, or is substituted with 13C, and particularly in that the Coenzyme A molecule is malonyl CoA and the structural analog is acetyl CoA-dg, methylmalonyl CoA-ds, methylmalonyl CoA-d4, propionyl CoA-d3, propionyl CoA-d5 or malonyl CoA-13C3. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a chromatogram for assay using a muscle sample from a Wistar rat. Fig. 2 is a chromatogram for assay using a liver sample from a Wistar rat. Fig. 3 is a chromatogram for assay using a brain sample from a Wistar rat. BEST MODE FOR CARRYING OUT THE INVENTION The invention relates to a method for assaying CoA molecules, which comprises extraction from a biological sample with a strongly acidic solution, followed by concentration if necessary, addition of an internal standard substance, preparation of an HPLC injection sample, HPLC separation of the CoA molecule and the internal standard substance, detection of the CoA molecule and the internal standard substance with a mass spectrometer, and quantitation from the area ratio of the detected CoA to be assayed and the internal standard substance. Extraction of the CoA molecule from a biological sample a strongly acidic solution may be performed from any biological sample containing a CoA molecule, and as specific examples there may be mentioned human and animal organs, human, animal and plant tissues or cells, microbes, and the like. Assay of CoA molecules in organs such as muscle, liver and brain is particularly useful. As CoA molecules to be assayed there may be mentioned acyl CoA molecules having acylated thiol groups, oxidized CoA molecules having oxidatively bonded thiol groups, and N-acyl CoA molecules having the primary amine acylated. Although some of the CoA molecules mentioned here are not found in biological samples, they are used for the purpose of research such as functional analysis and are therefore important for evaluating effects in drug development. As specific examples of acyl CoA molecules there may be mentioned acetoacetyl CoA, malonyl CoA, succinyl CoA, 3-hydroxy-3-methylglutaryl CoA, glutaryl CoA, CoA, acetyl CoA, benzoyl CoA, phenylacetyl CoA, isobutyryl CoA, isovaleryl CoA, butyryl CoA, betamethylcrotonyl CoA, tiglyl CoA, 3-hydroxypropionyl CoA, crotonyl CoA, hexanoyl CoA, methylmalonyl CoA, propionyl CoA, acryloyl CoA, arachidonyl CoA, decanoyl CoA, elaidoyl CoA, oleoyl CoA, palmitoleoyl CoA, palmitoyl CoA, linoleoyl CoA, lauroyl CoA, myristoleoyl CoA, nervonoyl CoA, stearoyl CoA, octanoyl CoA, myristoyl CoA, arachidonyl CoA, heptadecanoyl CoA, nonadecanoyl CoA, docosahexanoyl CoA, pentadecanoyl CoA, betahydroxybutyryl CoA, heptanoyl CoA, valeryl CoA, 2-butenoyl CoA and stearoyl CoA. As specific examples of N-acyl CoA molecules there may be mentioned N-butyryl CoA, N-decanoyl CoA and N-hexanoyl CoA. As specific examples of oxidized CoA molecules having oxidatively bonded thiol groups there may be mentioned oxidized CoA and CoA glutathione disulfide. Among these are preferred acetyl CoA, CoA, succinyl CoA, acetoacetyl CoA, 3-hydroxy-3-methylglutaryl CoA, propionyl CoA, methylmalonyl CoA, malonyl CoA, 3-hydroxypropionyl CoA, acryloyl CoA, oleoyl CoA, stearoyl CoA, linolenyl CoA, arachidonyl CoA, palmitoyl CoA, stearoyl CoA, isobutyryl CoA, oxidized CoA and CoA glutathione disulfide, with short chain (347, IS: m/z 831-»306) and the mass number of the monitor ion, and quantitation was conducted by the internal standard method based on the peak area ratio for malonyl CoA and IS. The calibration curve formula and the quantitative values for the QC sample and biological sample extracts were determined as follows. Calibration curve formula: Determined by the least square method (weighting: 1/x), using the relationship between the malonyl CoA peak area ratio (y) and concentration (x) with respect to IS. y = ax + b x: concentration, y: peak area ratio QC sample and biological sample extract concentrations: Determined by the following formula according to the calibration curve method, x = (y-b)/a x: Measured concentration 5. Results 1) Linearity: The calibration curve sample was measured according to the analysis method described above, and a calibration curve was created (Table 2). A weighting of 1/X was used, and the linearity was satisfactory with a correlation coefficient (r) of 0.9998 and a relative error (RE) of -7.5 to +6.8%. Table 2 Calibration curve data(Table Remove) 2) Reproducibility As samples for confirmation of reproducibility (QC), the malonyl CoA concentrations were assayed using a blank sample (QO) and QC samples containing 500 nM, 5 uM and 50 uM QC standard solutions (Ql, Q2, Q3, N=3 for each concentration). QC1 to QC3 represent the obtained quantitative values minus QO, and as shown in Tables 3 to 5, all of the values were satisfactory, with a purity (%CV) of 1.7-9.1% and a trueness (%RE) of -4.3 to 5.7% for the muscle concentration, a purity (%CV) of 3.0-8.2% and a trueness (%RE) of -5.6 to 9.1% for the liver concentration, and a purity (%CV) of 2.0-7.5% and a trueness (%RE) of -3.6-6.9% for the brain concentration. 3) Assay of malonyl CoA concentrations in biological samples Malonyl CoA concentrations were assayed in the muscle, liver and brain of six Wistar rats. The chromatograms are shown in Fig. 1 and Fig. 3, and the measurement results are shown in Table 6 and Table 8. Table 6 Malonyl CoA concentrations in Wistar rat muscle (Table Remove) EFFECT OF THE INVENTION According to the assay method of the invention, CoA molecules in biological samples can be quantitatively assayed in a precise and reproducible manner by LC-MS. Table 3 Results of reproducibility confirmation test using muscle samples(Table Remove) What is claimed is: A method for assaying concentrations of Coenzyme A molecules in biological samples, the method being characterized by comprising a step of extraction from a biological sample using a strongly acidic solution, a step of solid phase extraction, a step of adding an internal standard substance, and a step of detection by LOMS. The method for assaying Coenzyme A molecules according to claim 1, characterized in that the step of extracting the Coenzyme A molecule from the biological sample is a step wherein a freeze-shattered biological sample is agitated in a perchloric acid solution and the supernatant is subjected to centrifugal separation. The method for assaying Coenzyme A molecules according to claim 1 or 2, characterized in that the solid phase extraction step is a step wherein the supernatant obtained by extraction of the Coenzyme A with a strongly acidic solution is neutralized, and then applied to a reverse phase cartridge packed with silica gel containing an octadecylsilyl group or octylsilyl group, washed with an aqueous solvent, and eluted with an organic solvent. A . The method for assaying Coenzyme A molecules according to claim 3, characterized in that the supernatant is applied after conditioning the reverse phase cartridge with acetonitrile and 1 M ammonium acetate solution. The method for assaying Coenzyme A molecules according to claim 3, characterized in that the organic solvent is a mixture of acetonitrile and ammonium acetate. The method for assaying Coenzyme A molecules according to claim 1, characterized in that the Coenzyme A molecule is a fatty acid Coenzyme A ester, and the internal standard substance is a structural analog of the Coenzyme A molecule. The method for assaying Coenzyme A molecules according to claim 6, characterized in that the fatty acid Coenzyme A ester is a Coenzyme A ester of a short chain fatty acid with 2-8 carbons in the main carbon chain, and the structural analog has a difference of no more than 3 carbons with the Coenzyme A molecule and has at least 3 of the hydrogens of the main chain substituted with deuterium, or has at least 3 of the carbons of the main carbon chain substituted with 13C. 8. The method for assaying Coenzyme A molecules according to claim 7, characterized in that the Coenzyme A molecule is malonyl CoA and the structural analog is acetyl CoA-ds, methylmalonyl CoA-da, methylmalonyl CoA-d4, propionyl propionyl CoA-d5 or malonyl CoA-13C3.