Fat soluble and Water soluble vitamin


Classifying the Vitamins According to Their Solubilities

The vitamins are organic, low molecular weight substances that have key roles in metabolism. Few of the vitamins are single substances; almost all are families of chemically related substances, i.e., vitamers, sharing qualitatively (but not necessarily quantitatively) biological activities. Thus, the vitamers comprising a vitamin family may vary in biopotency, and the common vitamin name is actually a generic descriptor for all of the relevant vitamers.

Otherwise, vitamin families are chemically heterogeneous; therefore, it is convenient to consider their physical properties, which offer an empirical means of classifying the vitamins broadly.

The vitamins are frequently described according to their solubilities, that is, as being either fat-soluble or watersoluble. 1 Is interesting to note that this way of classifying the vitamins recapitulates the history of their discovery, calling to mind McCollum’s “fat-soluble A” and “watersoluble B.” The water-soluble vitamins tend to have one or more polar or ionizable groups (carboxyl, keto, hydroxyl, amino or phosphate), whereas the fat-soluble vitamins have predominantly aromatic and aliphatic characters.

Fat-soluble vitamins – appreciably soluble in non-polar

solvents:

Vitamin A Vitamin D

Vitamin E Vitamin K

Water-soluble vitamins – appreciably soluble in polar

solvents:

Thiamin Folate Vitamin B6

Niacin Vitamin C Pantothenic acid

Biotin Riboflavin Vitamin B12

The fat-soluble vitamins have some traits in common, in that each is composed either entirely or primarily of five-carbon isoprenoid units (i.e., related to isoprene, 2-methyl-1,3-butadiene) derived initially from acetyl-CoA in those plant and animal species capable of their biosynthesis. In contrast, the water-soluble vitamins have, in general, few similarities of structure. The routes of their biosyntheses in capable species do not share as many common pathways.

GENERAL PROPERTIES OF THE VITAMINS

Multiple Forms of Vitamins

Few of the vitamins are biologically active without metabolic conversion to another species and/or binding to a specific protein. Thus, any consideration of the vitamins in nutrition involves, for each vitamin group, a number of vitamers and metabolites; some of these are important in the practical sense for food and diet supplementation, whereas others are important in the physiological sense as they participate in metabolism.

Vitamin Stability

For the use of vitamins as food/feed additives, in diet supplements, and as pharmaceuticals, stability is a prime concern. In general, the fat-soluble vitamins, vitamin C, thiamin, riboflavin, and biotin are poorly stable to oxidation. They must be protected from heat, oxygen, metal ions (particularly Fe2and Cu2), polyunsaturated lipids undergoing peroxidation, and ultraviolet light; antioxidants are frequently used in their formulations. For vitamins A and E, the more stable esterified forms are used for these purposes. Because of the instabilities of their naturally occurring vitamers, the amounts of the fat-soluble vitamins in natural foods and feedstuffs are highly variable, being greatly affected by the conditions of food production and processing. Niacin, vitamin B6pantothenic acid, folate, and vitamin B12 tend to be more stable under most practical conditions.

 Vitamin Analysis

A variety of methods is available for the quantitative determination of the vitamins. Because many vitamers are bound to proteins or other factors in biological specimens and foods, their extraction necessitates disruption of those complexes and separation from interfering substances. This must be done in ways that are both quantitative and accommodate the intrinsic characteristics of each vitamer. Accordingly, conditions of sample extraction and clean-up must stabilize the vitamin(s) of interest in order to yield accurate results. Chromatographic separations have proven useful for determining vitamins A, D, E, K, C, thiamin, riboflavin, niacin, and vitamin B6.

They depend on separation by phase-partitioning (liquid– liquid,24 or gas–liquid25) of vitamers for specificity, ascertained by comparison to authentic standards, and a suitable means of detection (e.g., ultraviolet–visible absorption, fluorescence, electrochemical reactivity) for sensitivity. Microbiological assays are available for thiamin, riboflavin, niacin, vitamin B6, pantothenic acid, biotin, folate, and vitamin B12. These methods are based on the absolute requirement of certain microorganisms for particular vitamins for multiplication, which can be measured turbidimetrially or by the evolution of CO2 from substrate provided in the growth media. Some forms of vitamins A, E, and C can be measured by chemical colorimetric reactions; however, only the dye reduction methods for ascorbic acid have appropriate specificity and reliability to be recommended.

 

REFERENCE

The Vitamins

Fourth Edition

Gerald F. Combs, Jr

Professor Emeritus

Cornell University

Ithaca, NY

Post a Comment

0 Comments