Absorption, metabolism, and metabolic functions of pantothenic acid
About 85% of dietary pantothenic acid is as CoA and phosphopantetheine. In the intestinal lumen these are hydrolyzed to pantetheine; intestinal mucosal cells have a high pantetheinase activity and rapidly hydro-lyze pantetheine to pantothenic acid. The intestinal absorption of pantothenic acid seems to be by simple diffusion and occurs at a constant rate throughout the length of the small intestine; bacterial synthesis may contribute to pantothenic acid nutrition.
The first step in pantothenic acid utilization is phos-phorylation. Pantothenate kinase is rate limiting, so that, unlike vitamins that are accumulated by meta-bolic trapping, there can be significant accumulation of free pantothenic acid in tissues. It is then used for synthesis of CoA and the prosthetic group of acyl carrier protein. Pantothenic acid arising from the turn-over of CoA and acyl carrier protein may be either reused or excreted unchanged in the urine.
All tissues are capable of forming CoA from panto-thenic acid. CoA functions as the carrier of fatty acids, as thioesters, in mitochondrial β-oxidation. The resultant two-carbon fragments, as acetyl-CoA, then undergo oxidation in the citric acid cycle. CoA also functions as a carrier in the transfer of acetyl (and other fatty acyl) moieties in a variety of biosynthetic and catabolic reactions, including:
●cholesterol and steroid hormone synthesis
●long-chain fatty acid synthesis from palmitate and elongation of PUFAs in mitochondria
●acylation of serine, threonine and cysteine residues on proteolipids, and acetylation of neuraminic acid.
Fatty acid synthesis is catalyzed by a cytosolic multi-enzyme complex in which the growing fatty acyl chain is bound by thioester linkage to an enzyme-bound 4′-phosphopantetheine residue, rather than to free CoA, as in β-oxidation. This component of the fatty acid synthetase complex is the acyl carrier protein.