The biological origin of ketotic dicarboxylic aciduria: In vivo and in vitro investigations of the ω-oxidation of C 6–C 16-monocarboxylic acids in unstarved, starved and diabetic rats

Abstract

The conversion of radioactive C 6–C 16-monocarboxylic acids to urinary adipic, suberic, sebacic and 3-hydroxybutyric acids was investigated in vivo in unstarved, starved and diabetic ketotic rats. Hexanoic, octanoic and decanoic acids were converted to C 6-, C 6-C 8- and C 6-C 10-dicarboxylic acids, respectively, in fed and 72-h-starved rats. Lauric acid was converted to C 6–C 8-dicarboxylic acids in starved rats but not in unstarved rats. Decanoic and lauric acids were converted to relatively high amounts of C 6–C 8-dicarboxylic acids compared with myristic acid in ketotic diabetic rats, while radioactivity from [1- 14C]- and [16- 14C]palmitic acid was not incorporated into C 6–C 8-dicarboxylic acids in diabetic ketotic rats. C 6–C 12-monocarboxylic acids in hydrolysed rat adipose tissue were determined by gas-liquid chromatography-mass spectrometry (selected ion monitoring). Decanoic and lauric acids were found in amounts of 7.6–9.1 and 85.9–137.5 μg/100 mg tissue, respectively, whereas the amounts of hexanoic and octanoic acids were negligible. It is concluded that the biological origin of the C 6-C 8-dicarboxylic aciduria seen in ketotic rats are C 10-C 14-monocarboxylic acids, which are initially ω-oxidised solely or partly as free acids and subsequently β-oxidised to adipic and suberic acids. The in vitro ω-oxidation of C 6-C 16-monocarboxylic acids to corresponding dicarboxylic acids in the 10 000 × g supernatant fraction of rat liver homogenate was measured by selected ion monitoring. 0.09 0.14,16.1 5.8 7.0 and −6.9% of, respectively, hexanoic, octanoic, decanoic, lauric, myristic and palmitic acid were ω-oxidised to dicarboxylic acids of corresponding chain lengths after 90 min of incubation, when correction for the production of dicarboxylic acids in control assays was made. An in vitro production of C 12-C 16-dicarboxylic acids was detected in all assays (including control assays), probably formed from ‘endogenous’ monocarboxylic acids preexistent in the homogenate. This ‘endogenous’ production of dicarboxylic acids was inhibited by C 10-C 16-monocarboxylic acids, where palmitic acid had the strongest effect. In fact, palmitic acid inhibited its own ω-oxidation when added in concentrations above 0.6 mM. Starvation of rats for 72 h did not alter the ‘endogenous’ in vitro production of hexadecanedioic acid.