Maize Microsomal Benzoxazinone N-Monooxygenase

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Abstract
The benzoxazinones occur in hydroxamic acid and lactam forms in maize (Zea mays L.) tissue. The hydroxamic acid forms which possess a N-hydroxyl group are found in the highest concentration while the lactam members which lack the N-hydroxyl group occur in lower concentrations. The hydroxamic acid 2,4-dihydroxy-1,4-benzoxazin-3-one (DIBOA) has as its lactam counterpart 2-hydroxy-1,4-benzoxazin-3-one (HBOA). An enzyme has been identified in maize microsomal preparations which catalyzes the N-hydroxylation of HBOA to form DIBOA. The enzyme is initially observed in seedlings 2 days after imbibition which coincides with the onset of hydroxamic acid accumulation. The enzyme requires NADPH and is inhibited by sulfhydryl reagents, NADP, cytochrome c, cations, carbon monoxide, and nitrogen gas. The effect of nitrogen can be reversed by exposing the enzyme to air, while the effect of carbon monoxide can be reversed by exposing the enzyme to 450 nanometer light during the incubation period. The apparent Km values for HBOA and NADPH are 13 and 5 micromolar, respectively. The pH optimum is 7.5 and the temperature optimum for the enzyme is 35°C. A 450 nanometer absorbance peak is observed when reduced microsomal preparations are exposed to carbon monoxide which in combination with other data presented supports the hypothesis that the enzyme is a cytochrome P-450 dependent N-monooxygenase.