The main purpose of this projected study is to investigate the structure and regulation of 4-aminobutyrate aminotransferase, the enzyme involved in the catabolism of the neurotransmitter GABA. The structural studies entail the elucidation of the primary sequence of the enzyme by either direct amino acid sequence or by analysis of the C-DNA encoding this aminotransferase. Valuable information on certain regions of the polypeptide chain modified by specific chemical reagents have been obtained by direct amino groups of the enzyme has been determined together with the identification of the SH pair undergoing oxidation/reduction. Tryptic peptides isolated and purified by HPLC chromatography will be used in further sequential studies. It is expected that the complete amino acid sequence of the enzyme will be elucidated in less than two years. Screening of a library of C-DNA clones inserted into the phage lambda G 11 has begun. Two methods, i.e., hybridization with oligodeoxyribonucleotide probes and recognition by antibody probes are being used in the screening steps. The results derived from the sequential studies are important for further analysis of specific regions of the aminotransferase by site- directed mutagenesis.