Organic Acid and Acylcarnitine Analysis
Urine organic acid analysis revealed that MCAD−/− mice developed an organic acid profile similar to MCAD-deficient human patients. Specifically, when fasted for 18 h, MCAD−/− mice developed significantly elevated concentrations of adipic, suberic, and sebacic acids and hexanoylglycine as compared to MCAD+/+ controls, which showed trace to no detectable amounts of the same organic acids (Table 1). Adipic acid is not specific to MCAD deficiency. We also evaluated β-hydroxybutyric and acetoacetic concentrations and found no significant differences between MCAD genotypes (data not shown).
Comparison of MCAD+/+ and MCAD−/− mice revealed no significant differences in total serum carnitine concentrations between MCAD+/+ and MCAD−/− mice, but MCAD−/− mice had a 5- to 6-fold elevation of serum decenoylcarnitine evident in the acylcarnitine profile (Figure 4A). Bile acylcarnitine analysis revealed a similar acylcarnitine pattern as in serum (Figure 4B). However, the acylcarnitine profiles of the MCAD−/− mice are different from those of human MCAD-deficient patients (Figure 4C). Human patients present with elevated levels of C6, C8, and C10:1 acylcarnitines, as did the mutant mice; however, the predominant peak was C8 acylcarnitine in humans, whereas in the mouse it was C10:1 acylcarnitine.
Figure 4  Acylcarnitine Analyses
(A) Serum acylcarnitine analysis of MCAD+/+ (n = 4) and MCAD−/− mice (n = 4)
There are significant elevations in acylcarnitine species as indicated in MCAD−/− mice. Values shown are mean values ± standard deviation (SD). Asterisk indicates p < 0.002 and ‡ indicates p < 0.01.
(B) There are significant elevations in bile acylcarnitines of the same mice shown in (A) as indicated. Values shown are mean values ± SD. Asterisk indicates p < 0.001.
(C) Bile acylcarnitine profile of an MCAD−/− mouse compared to a human patient with MCAD deficiency. Internal standards are indicated by an asterisk.