Differentiation between the 2 disorders requires measurement of LCTH activity.
To date, isolated LCTH deficiency associated with HADHB mutations has not been reported.
To confirm the diagnosis of isolated LCTH deficiency, we analyzed the HADHA and HADHB genes coding for MTP in cultured fibroblasts.
We report the first case of isolated LCTH deficiency resulting from mutations of the HADHB gene in a male newborn.
The phenylalanine at position 431 is not conserved in yeast peroxisomal thiolase but is located near the catalytic histidine at position 429 and apparently affects the catalytic activity of LCTH rather than the stability.
 Nonstandard abbreviations: MTP, mitochondrial trifunctional protein; HADHA and HADHB, hydroacyl-CoA dehydrogenase-[alpha] and -[beta] subunits, respectively; LCEH, long-chain enoyl-CoA hydratase; LCHAD, long-chain 3-hydroxyacyl Co-A dehydrogenase; LCTH, long-chain ketoacyl-CoA thiolase; C160H, hydroxypalmitoylcarnitine; C16:1OH, hydroxyhexadecenoylcarnitine; C180H, hydroxyoctadecanoylcamitine; C18:1OH, hydroxyoctadecenoylcarnitine; MS/MS, tandem mass spectrometry; C14:1, hexadecenoylcarnitine; C16, palmitoylcarnitine; C16:1, hexadecenoylcarnitine; and C2, acetylcamitine.