Normal brain myelination in a patient homozygous for a mutation that encodes a severely truncated methionine adenosyltransferase I/III.

Two isozymes of mammalian methionine adenosyltransferase, MAT I and MAT III, are expressed solely in adult liver. They are, respectively, tetramers and dimers of a single subunit encoded by the gene MAT1A. A third isozyme, MAT II, contains a catalytic subunit encoded by a separate gene, MAT2A, and is expressed in a variety of tissues, including (to a slight extent) adult liver. Based on a recent finding that 2 children with isolated hypermethioninemia and brain demyelination were homozygous for MAT1A mutations predicted to produce severely truncated proteins, and devoid of activity when expressed, it was concluded that complete lack of MAT I/III activity may be associated with neurological symptoms and demyelination. We now report that a 43-year-old man with persistent isolated hypermethioninemia, previously demonstrated to have deficient MAT activity in his liver, has normal brain myelination on MRI and normal neurological function, despite being homozygous for a 539 TG insertion in exon V of MAT1A, so that the gene is predicted to encode a protein of only 184 rather than the normal 395 amino acids. This patient's exon V mutation was demonstrated by SSCP analysis and verified by sequencing. Both parents are heterozygous for the same insertion. This suggests that MAT1A mutations producing severely truncated proteins do not necessarily produce brain demyelination. This finding has relevance to a previously reported 4-year-old girl who was also homozygous for the 539insTG mutation. Finally, our patient's 7% residual hepatic MAT activity, measured at 1 mM methionine, may reflect the hepatic activity of the more ubiquitous enzyme form, MAT II.

Measurement of IR laser backscatter spectra from sulfuric acid and ammonium sulfate aerosols.

An optical system employing a tunable carbon dioxide laser has been used to investigate backscatter signatures of aerosols as a function of wavelength. Submicron sulfuric acid or ammonium sulfate aerosols are produced with a vapor-condensation aerosol generator. The aerosol is contained in a 1-m long windowless aerosol chamber, and laser radiation backscattered from the irradiated aerosol is collected and measured as the laser is tuned from 9.2 to 10.8 microm. The volume backscatter coefficient is calculated from the lidar equation to yield the theoretical IR spectrum of the aerosol. The measured spectral signature is compared with the theoretical signature, which is computed from Mie theory. Backscatter signatures show excellent agreement with calculated signatures. The spectral signature of ammonium sulfate is readily distinguished from that of sulfuric acid for the conditions of the experiment. Because of vapor pressure characteristics of sulfuric acid, it is possible to concentrate the acid in the generator over time and look for a change in the acid concentration in the aerosol. Not only has this concentration process been observed optically, but under these experimental conditions the acid concentration in the aerosol can be determined by observing backscatter at just two wavelengths.