Publisher Summary This chapter discusses the reactions involved in the formation of palmitic and stearic acids. It discusses the biosynthesis of saturated fatty acids in plants. There is increasing evidence that in plant tissues, the synthesis of palmitic acid involves a de novo system; it is so called because it utilizes acetyl-CoA, malonyl-CoA, acyl carrier protein (ACP), and a battery of soluble, nonassociated enzymes to form, as its terminal product, palmitoyl-ACP. Palmitoyl-ACP is then elongated by another set of enzymes called the C16– C18 elongation system that converts palmitoyl-ACP to stearoyl-ACP with malonyl-ACP as the condensing C2 unit. In recent years, increasing evidence has suggested that, unlike prokaryotes and animals, higher plants have unique organelles that appear to be specific sites of the de novo and elongation systems as well as of the first desaturation step that utilizes stearoyl-ACP as the specific substrate for the introduction of the first double bond in the 9,10-position to form oleoyl-ACP. In leaves, the chloroplast is the specific site; in seeds, the proplastid is the specific site.

Biosynthesis of Saturated and Unsaturated Fatty Acids

A highly biologically active metabolite of retinoic acid (8III) has been isolated in pure form from intestinal mucosa of vitamin A deficient rats given [3H]retinoic acid. This metabolite has been positively identified as 5, 6-epoxyretinoic acid based on the ultraviolet absorption spectrum and mass spectrum of its methylated derivative. This identification was confirmed by cochromatography of the methylated metabolite and synthetic methyl 5, 6-epoxyretinoate on reverse-phase and straight-phase high-pressure liquid chromatography. The 5, 6-epoxyretinoic acid is a true in vivo generated metabolite of retinoic acid and not an artifact of the isolation procedure. In addition, 5, 8-oxyretinoic acid previously isolated in this laboratory from intestinal mucosa was probably generated from 5, 6-epoxyretinoic acid by the acidic conditions used in the extraction and isolation of the 5, 8-oxyretinoic acid.

Isolation and identification of 5, 6-epoxyretinoic acid: a biologically active metabolite of retinoic acid.

Identification of 5,6-epoxyretinoic acid as an endogenous retinol metabolite.

In vitro and in vivo metabolism of all-trans- and 13-cis-retinoic acid in hamsters. Identification of 13-cis-4-oxoretinoic acid.

Plasma retinoid profile in bullfrogs, Rana catesbeiana, in relation to agricultural intensity of sub-watersheds in the Yamaska River drainage basin, Québec, Canada.

The low resolution mass spectra of 5,6-monoepoxyretinoic acid, 5,8-monoepoxyretinoic acid, 5,6-monoepoxyretinyl acetate, 5,8-monoepoxyretinyl acetate, 5,8-monoepoxyretinal, trimethylsilyl-5,6-monoepoxyretinoate, trimethylsilyl-5,8-monoepoxyretinoate, methyl-5,6-monoepoxyretinoate, methyl-5,8-monoepoxyretinoate, β-C19-retinal and β-C19-retinyl acetate-analogs of vitamin A—are presented. Characteristic features of the spectra are discussed. In addition, high resolution mass measurements are presented for six fragment ions of all-trans-retinoic acid and eleven fragment ions of methyl-5,6-monoepoxyretinoate.

Mass spectral analysis of eleven analogs of vitamin A.

A mixture of all-trans-retinoic acid and iodine in heptane was irradiated. Two oxidation products were isolated by high performance liquid chromatography and identified as all-trans- and 13-cis-4-oxoretinoic acid by nuclear magnetic reasonance, ultra violet, Infrared spectroscopy, and mass spectral analysis. Under the same conditions, but without light, a mixture of all-trans- and 13-cis-retinoic acid resulted. The corresponding methyl esters were obtained when methyl all-trans-retinoate was used in place of all-trans-retinoic acid.

M. L. Mcgregor

Papers

Mass spectral analysis of eleven analogs of vitamin A.

Oxidation and isomerization of retinoic acid by iodine and light: A novel preparation of all-trans- and 13-cis-4-oxoretinoic acid