SLC 16A2, the gene for the second member of the solute carrier family 16 (monocarboxylic acid transporter), located on chromosome Xq13.2, encodes a very efficient thyroid hormone transporter: monocarboxylate transporter 8, MCT8. Its loss of function is responsible in males for a continuum of psychomotor retardation ranging from severe (no motor acquisition, no speech) to mild (ability to walk with help and a few words of speech). Triiodothyronine uptake measurement in transfected cells and, more recently, patient fibroblasts, has been described to study the functional consequences of MCT8 mutations. Here, we describe three novel MCT8 mutations, including one missense variation not clearly predicted to be damaging but found in a severely affected patient. Functional studies in fibroblasts and JEG3 cells demonstrate the usefulness of both cellular models in validating the deleterious effects of a new MCT8 mutation if there is still a doubt as to its pathogenicity. Moreover, the screening of fibroblasts from a large number of patient fibroblasts and of transfected mutations has allowed us to demonstrate that JEG3 transfected cells are more relevant than fibroblasts in revealing a genotype-phenotype correlation.

Glutamate dehydrogenase (GDH) activity was studied in 17 regions of six human brains. Duration and conditions of the postmortem period did not affect enzyme activity. Specific activity ranged between 103 and 377 nmoles/min/mg protein at 25 degrees C and it was 10-fold higher than that found in leukocytes. Apart from exclusively white matter regions (corpus callosum and centrum ovale), there was a moderate regional distribution (2.5-fold variation), with highest values in the inferior olive and hypothalamus, and lowest in the cerebellum and lenticular nucleus. With alpha-ketoglutarate (alpha-KG), NADH, or NH4+ as variable substrate, the apparent Km values in human brain were Km alpha-KG = 1.9 X 10(-3) M, KmNADH = 0.21 X 10(-3) M, and KmNH4+ = 28 X 10(-3) M, and in leukocytes they were Km alpha-KG = 1.7 X 10(-3) M, KmNADH = 0.24 X 10(-3) M, and KmNH4+ = 28 X 10(-3) M. The effects of cofactors, inhibitor, and pH were similar in brain and leukocyte GDH.

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