Trihydroxycholestanoic acid THCA

Trihydroxycholestanoic acid (THCA) Dihydroxycholestanoic acid (DHCA) Phytanic acid Pristanic acid... 

Very-long-chain fatty acids (VLCFA), trihydroxycholestanoic acid (THCA), phytanic acid and pristanic acid can all be measured in plasma/serum (> 1 ml). From the same blood sample erythrocytes, platelets and/or leukocytes can be prepared for determination of plasmalogens and dihydroxyacetone-phosphate acyltransferase (DHAPAT), respectively. 

Figure 4 Peroxisomal fatty-acid (FA) /3-oxidation pathways. While saturated long-chain fatty acids (LCFA) are preferentially degrade in mitochondria, saturated very-long-chain fatty acids (VLCFA) and some LCFA are shortened by peroxisomal /3-oxidation. Degradation of pristanic acid, the product of phytanic acid a-oxidation, and the conversion of the cholesterol-derived 27-carbon bile-acid precursors dihydroxycholestanoic acid (DHCA) and trihydroxycholestanoic acid (THCA) to 24-carbon bile acids also require this pathway. The mechanism by which these substrates enter peroxisomes is unknown. Four enzymatic reactions serve to shorten the substrates by either two (LCFA, VLCFA) or three (pristanic acid, DHCA, THCA) carbon atoms. The 2-methyl group of the latter substrates is shown in brackets. SCPx thiolase refers to the thiolase activity of sterol carrier protein x.

Alkyl PAT, alkyl-dihydroxy phosphate synthase Bif, bifunctional enzyme DHAPAT, dihydroxyphosphate acyltransferase deficiency DHCA, dihydroxycholestanoic acid N, normal nd, not determined Ox, acyl-CoA oxidase Rac, 2-methylacyl-CoA racemase RCDP, rhizomelic chondrodysplasia punctata Ref, Refsum s disease THCA, trihydroxycholestanoic acid VLCFA, very-long-chain fatty acid. 

The most important substrates handled by the peroxisomal fatty acid oxidation system from the perspective of peroxisomal disorders are (1) very-long-chain fatty acids (VLCFA), notably hexacosanoic acid (C26 0), (2) pris-tanic acid (2,6,10,14-tetramethylpentadecanoic acid), as derived from dietary sources either directly or indirectly from phytanic acid and (3) di- and trihydroxycholestanoic acid (DHCA and THCA). The latter two compounds are intermediates in the formation of the primary bile acids cholate and chenodeoxycholate from cholesterol in the liver. 

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