Glutamate 1-semialdehyde aminotransferase (glutamate 1-semialdehyde 2,1-aminomutase; EC 5.4.3.8; GSA-AT) catalyzes the transfer of the amino group on carbon 2 of glutamate 1-semialdehyde (GSA) to the neighboring carbon 1 to form delta-aminolevulinic acid (ALA). To gain insight into the mechanism of this enzyme, possible intermediates were tested with purified enzyme and the reaction sequence was followed spectroscopically. While 4,5-dioxovaleric acid (DOVA) was efficiently converted to ALA by the pyridoxamine 5'-phosphate (PMP) form of the enzyme, 4,5-diaminovaleric acid (DAVA) was a substrate for the pyridoxal 5'-phosphate (PLP) form of GSA-AT. Thus, both substances are reaction intermediates. The purified enzyme showed an absorption spectrum with a peak around 338 nm. Addition of PLP led to increased absorption at 338 nm and a new peak around 438 nm. Incubation of the purified enzyme with PMP resulted in an additional absorption peak at 350 nm. The reaction of the PLP and PMP form of the enzyme with GSA allowed the detection of a series of peaks which varied in their intensities in a time-dependent manner. The most drastic changes to the spectrum that were observed during the reaction sequence were at 495 and 540 nm. Some of the detected absorption bands during GSA-AT catalysis were previously described for several other aminotransferases, indicating the relationship of the mechanisms. The reaction of the PMP form of the enzyme with DOVA resulted in a similar spectrum as described above, while the spectrum for the conversion of DAVA by the PLP form of the enzyme indicated a different mechanism.(ABSTRACT TRUNCATED AT 250 WORDS)