Fungal ornithine esterases: relationship to iron transport.

TLDR: It is proposed that these specific ornithylesterases provide a mechanism of cellular iron release by hydrolyzing of the ferric ionophores, and that an iron-exchange step occurs prior to, and is a prerequisite for, hydrolysis of the ester bonds.

Abstract: Extracts of Fusarium roseum (ATCC 12822) contain an enzyme which hydrolyzes the ornithine ester bonds of fusarinine C, a cyclic trihydroxamic acid produced by this organism The methyl ester of Ndelta-dinitrophenyl-L-ornithine is also a substrate for the enzyme, and an assay was devised using this substrate The enzyme exhibits a sharp maximum of activity at pH 75 and is extremely temperature sensitive It is strongly inhibited by HgCl2 and p-chloromercuribenzoate, and it is competitively inhibited by Ndelta-dinitrophenyl-D-ornithine methyl ester (Ki = 03mM) Methyl esters of glycine, L-alanine, dinitrophenyl-L-alanine, dinitrophenyl-beta-alanine, and Ndelta-dinitrophenyl-Nalpha-acetyl-L-ornithine are not substrates, although Nepsilon-dinitrophenyl-L-lysine methyl ester is as effective as the ornithine derivative Nonspecific lipases do not hydrolyze ornithine esters, nor does trypsin The three ester bonds of fusarinine C are progressively hydrolyzed by the enzyme to eventually yield the monomer, fusarinine The ferric chelate of fusarinine C is not hydrolyzed An enzyme from Penicillium sp was isolated with identical properties toward Nbeta-dinitro-phenyl-L-ornithine methyl ester as substrate It also hydrolyzes N,N',N"-triacetylfusarinine C, a cyclic trihydroxamate containing Nalpha-acetylornithine ester bonds, which is produced by this organism This substrate is hydrolyzed to Nalpha-acetylfusarine In contrast to the Fusarium enzyme, this enzyme is fully active toward the ferric trihydroxamate chelate However, replacement of iron by aluminum leads to a completely inactive substrate Production of the enzyme is severely suppressed by iron in the growth medium It is proposed that these specific ornithylesterases provide a mechanism of cellular iron release by hydrolysis of the ferric ionophores, and that an iron-exchange step occurs prior to, and is a prerequisite for, hydrolysis of the ester bonds