Abstract
The accumulation and excretion of citric acid by Aspergillus niger is a notable example of fungal 'over-flow' metabolism, and for 30 years has been the world's primary source of citric acid. However, the biochemical information available on this process has to date not permitted its elucidation, but recently certain regulatory steps of A. niger energy metabolism have been investigated with respect to citric acid accumulation [ 1-4]. Malate dehydrogenase (EC 1.1.1.37) isoenzymes of A. niger were investigated because of the key role of oxaloacetate in the accumulation of citric acid [5]. They are involved in a shuttle mechanism which transports reduced NADH across the mitochondrial membrane and thereby generates mitochondrial oxaloacetate. On the other hand, there is a very active pyruvate carboxylase (EC 6.4.1.1), located intramitochondrially [6], whose activity in vivo has been shown to be strongly correlated with citric acid accumulation [7]. The present paper reports some molecular and regulatory properties of two malate dehydrogenase isoenzymes from A. niger. 2.1. Organism and conditions for growth
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