Studies on the conversion of the cytosol delta-aminolevulinate synthetase to the mitochondrial enzyme.

DELTA-Aminolevulinate (ALA) synthetase was induced in the rat by allylisopropylacetamide, and ALA synthetase in the liver cytosol fraction was partially purified. The cytosol ALA synthetase shows a molecular weight (M.W.) of about 600,000 by gel filtration and about 178,000 by sucrose density gradient centrifugation, whereas the mitochondrial enzyme gives M.W. about 110,000 by either method. In the presence of 0.25 M NaCl, however, the cytosol ALA synthetase showed M.W. 300,000--350,000 by gel filtration and gave M.W. 110,000 or a S value of 6.4 by centrifugation. Treatment of the cytosol ALA synthetase with papain or snake venom also yielded an entity which showed M.W. 110,000 by gel filtration as well as by sucrose density gradient centrifugation. When the 6.4 S enzyme once separated by centrifugation was combined with a fraction exhibiting about 4 S on centrifugation and was subsequently subjected to gel filtration, the ALA synthetase activity was recovered in the fraction corresponding to M.W. about 170,000. When the combination of the 6.4 S enzyme and a fraction of about 8 S was examined by gel filtration, the ALA synthetase activity was eluted showing M.W. about 250,000. The ALA synthetase in the cytosol fraction seems to exist as a complex composed of an ALA synthetase of M.W. 110,000 and a 4 S protein and an 8 S protein to form a larger size of the enzyme (M.W. 300,000) which further aggregates to give M.W. about 600,000. Physiologically, the complex may be subjected to a limited proteolysis so as to lose the capacity to form complex and aggregates, followed by incorporation of smaller sizes of the enzyme into the mitochondria.