Alternative Routes for the Synthesis of 5-Aminolevulinic Acid in Maize Leaves

Abstract

Cell-free extracts from greening maize (Zea mays L.) leaves catalyze the conversion of I'4C]2-ketoglutarate (KG) to 14C15-aminolevulinic acid (ALA) in a reaction which requires NADH and an amino donor and shows maximal activity around pH 6.5. The enzymic system is located in the cytosol. This cell fraction contains a low level of 'KG dehydrogenase' activity and a transaminase which catalyzes the conversion of 4,5-dioxovaleric acid (DOVA) to ALA. The transaminase can use glutamate, aspartate, or alanine as amino donor. It is effectively inhibited by aminooxyacetate and ethylenediamine tetraacetate and shows maximal activity at pH 6.7. The activity of DOVA transaminase is only sllghdy affected by preillumination of leaves and can also be detected in green leaves and in roots.DOVA was isolated from leaves and roots and determined as its benzoquinoxaline derivative. Significant amounts were found only in tissues in which ALA had accumulated or after it was exogenously supplied.DOVA was labeled in vivo by both 114CIALA and I14CIKG. Small amounts were also formed from ALA in a cell-free system. It is suggested that DOVA may be an intermediate in the diversion of ALA to respiratory metabollsm and that it is not involved in the biosynthesis of this porphyrin precursor.

Alternative Routes for the Synthesis of 5-Aminolevulinic Acid in Maize Leaves

“…It strongly inhibited the formation of ALA from both glutamate and KG (Table I) (14). On the whole, the results shown in Table I strengthen the conclusion that isolated plastids convert KG to ALA through first transforming it to glutamate and not to DOVA (13).…”

“…Previous failure to demonstrate it was apparently due to difficulties in obtaining intact plastids from greening maize leaves and the marked sensitivity of the enzyme system to inactivation. The work lends further support to the conclusion of a preceding paper (13) that ALA is synthesized in greening maize leaves from glutamate and not from KG and DOVA, and describes some properties of the system. A preliminary report of some of its observations has been published as an abstract (23 (13).…”

“…The work lends further support to the conclusion of a preceding paper (13) that ALA is synthesized in greening maize leaves from glutamate and not from KG and DOVA, and describes some properties of the system. A preliminary report of some of its observations has been published as an abstract (23 (13). Barley (Hordeum vulgare, cv Svalofs Bonus) seeds were germinated and grown as previously described (1 1).…”

“…In a preceding paper (13) we concluded that the conversion of KG to ALA by intact maize plastids was apparently due to their ability to readily convert KG to glutamate. This conversion can take place by several independent enzymes, all of which have been reported to occur in plastids from greening maize leaves (10,26 .…”

A Low Molecular Weight Polypeptide Which Accumulates upon Inhibition of Porphyrin Biosynthesis in Maize

“…This is apparent from the diversity of the compounds which induce LMWP formation-the only property common to all being the ability to interfere with porphyrin synthesis. The fact that fl-hydroxyglutamate does not induce the formation of LMWP supports the conclusion that the latter is not related to the synthesis of plastid porphyrins, which has been shown to proceed via the glutamate:glutamate-1 -semialdehyde route of ALA synthesis (16,17,30). Accumulating evidence suggests that extraplastidial porphyrins are synthesized by ALA synthase from succinyl-CoA and glycine (6,13,23).…”