In the patients of group A, both carnitine (C0) and acetylcarnitine (C2) are significantly lower than in patients in group B; however, the concentrations of other acylcarnitines are all significantly higher in group A than that in group B. This suggests that the activity of carnitine acetyltransferase

more is significantly lower in group B and there are differences in fatty acid metabolism between these two groups. Elevated acylcarnitine levels have been detected in obesity,22 type-2 diabetes,23 cardiovascular disease24 and encephalopathy.25 Group B can be classified into group B1 and B2 based on the HCA. The significant metabolites that contribute to these subgroups are mainly glycerophospholipids, sphingolipids and amino acids The physiological significance regarding OA for these kinds of metabolites has been previously studied. Glycerophospholipids form the essential lipid bilayer of all biological membranes and are intimately involved in signal transduction, regulation of membrane trafficking and many other membrane-related phenomena.26

27 Studies by Hills28 indicated that alterations in phospholipid composition and concentrations are associated with the development of OA. Kosinska et al29 also found that in comparison with control SF, the levels of glycerophospholipids (Five phosphatidylglycerol and two lysophosphatidylglycerol species) were all elevated in late OA by 3.6-fold. Our results are consistent with their findings. The concentrations of 24 glycerophospholipids in patients of group B2 were all significantly higher than those in patients of group B1, especially for the PC types. Sphingolipids are a class of lipids that include ceramide species, sphingomyelins (SMs) and more complex glycosphingolipids, which are an important part of SF. Sphingolipids are structural components of plasma membranes and bioactive molecules that have significant functions in proliferation and growth as well as differentiation, cellular signal transduction and apoptosis in many mammalian cells, for instance,

fibroblast-like synoviocytes and neural cells.30–33 Studies by Marta Entinostat et al29 suggest that SM species had risen approximately twofold in SF from early OA to late OA. In our study, the concentration of nine significant sphingolipids (6 SM, 3 SM(OH)) in group B2 was significantly higher than that in group B1 by about 1.7 fold. In metabolic disorders, the knowledge of the concentration of one amino acid or related group of amino acids is essential for correct diagnosis. For example, the cellular energy metabolism accessed by amino acids profiling can be used for in-depth analysis of chronic fatigue syndrome.34 The branched-chain amino acids (BCAA), valine, isoleucine and leucine, are essential amino acids, accounting for 35% of the essential amino acids in muscle proteins.