Fatty Acid Synthesis by Extracts of Euglena

Abstract

At least 3 known pathways are available to a cell to synithesize satuirated fatty acids: A) the malonylCoA pathwav which has been shown to result in do novo synthesis of long-chain fatty acids, B) reversal of the fl-oxidation pathway which results primarily in syinthesis of short-chain acids and onlv negligible amounlits of long-chain fatty acids. and C) a combination of these 2 processes by which lprodutcts of the malonyl-CoA system may be elongated presumably by enzymes of the :-oxidation pathway. Distinctive differences exist between these pathways for fatty acid synthesis. They differ both in the nature of the products formed and in the cofactors required when acetyl-CoA is the stubstrate. \Vhereas ATP, HCO3-, Mn+ and TPNH are required in the malonyl-CoA system only TPNH and DPNH are required for reversal of the f-oxidation pathway. Both of these systems have been wvell sttudied in animal tissue, bacteria, and yeast but to a less extent in higher plants. Very little investigation has been done with algae despite the fact that it has long been recognized that these organisms have a diverse spectrtum of both saturated and utnsaturated fatty acids and that the amotunit of unsaturation is somewhat dependent upon cutltture unlder photosynthetic conditions (12.28). In ouir past studies with extracts of the alga Enuglenci (10,11), evidence has been presented showing the requirement for ATP, Mg+, DPNH, and TPNH for incorporation of acetyl-CoA into longchain fatty acids. Stoichiometric studies have given results analogous to those obtained in the malonylCoA systemn, i.e., 1 ATP and 2 reduced pyridine nucleotides are tutilized per acetyl-CoA incorporate(d into fatty acids (60-70 % of 16-20 carbon atoims). Bicarbonate wlhiclh is essential for fatty acid synithesis via the imalonyl-CoA pathway is, however, not required, and the biotin-binding protein avidin is not inhibitory. These observations and the fact that the rate of incorporationi of malonyl-CoA is infinitesimally smiiall compared to that of acetyl-CoA would suggest that we are not dealing with fatty acid synthesis via the malonyl-CoA pathway. We wish to report here on further studies of the products and certain other properties of the system. These studies include the effect of avidin on distribution of products, the distribution of C'4 in stearate biosynthesized from acetyl-1-C'4-CoA, and the chemical nature of the products. In addition, we shall give data on the sensitivity of the fatty acid synthesizing system to various -SH inhibitors and the specificity of the system for nucleoside triphosphates.