Marginal Compounds and Phytonutrients

In addition to the established vitamins, a number of organic compounds have clear metabolic functions; they can be synthesized in the body, but it is possible that under some circumstances (as in premature infants and patients maintained on long-term total parenteral nutrition) endogenous synthesis may not be adequate to meet requirements. These compounds include biopterin (Section 10.4), carnitine (Section 14.1), choline (Section 14.2), creatine (Section 14.3), inositol (Section 14.4), molybdopterin (Section 10.5), taurine (Section 14.5), and ubiquinone (Section 14.6).

A number of compounds found in foods of plant origin have potentially protective effects, although they cannot be considered to be dietary essentials; they are variously known as phytonutrients, phytoceuticals, or nutraceuticals. Such compounds include allyl sulfur compounds, flavonoids, glucosinolates, and phytoestrogens.

14.1 CARNITINE

Carnitine (3-hydroxy,4-N-trimethylaminobutyric acid) has a central role in the transport of fatty acids across the mitochondrial membrane for ß -oxidation. At the outer face of the outer mitochondrial membrane, carnitine acyltransferase I catalyzes the reaction shown in Figure 14.1, the transfer of fatty acids from coenzyme A (CoA) to form acyl carnitine esters that cross into the mitochondrial matrix. At the inner face of the inner mitochondrial membrane, carnitine acyltransferase II catalyzes the reverse reaction.

Acyl carnitine can cross only the inner mitochondrial membrane on a coun-tertransport system that takes in acyl carnitine in exchange for free carnitine being returned to the intermembrane space. Once inside the mitochondrial inner membrane, acyl carnitine transfers the acyl group onto CoA ready to undergo ß-oxidation. This countertransport system provides regulation of the

CoASH

CoASH

SCoA

cooh

SCoA

carnitine acyltransferases ch2 I

COOH

Figure 14.1. Reaction of carnitine acyltransferase (carnitine palmitoyltransferase, EC 2.3.1.21).

uptake of fatty acids into the mitochondrion for oxidation. As long as there is free CoA available in the mitochondrial matrix, fatty acids can be taken up and the carnitine returned to the outer membrane for uptake of more fatty acids. However, if most of the CoA in the mitochondrion is acylated, then there is no need for further fatty uptake immediately, and it is not possible.

Carnitine acyltransferase I is strongly inhibited by malonyl CoA, and muscle has both acetyl CoA carboxylase, which forms malonyl CoA, and malonyl CoA decarboxylase, which acts to remove malonyl CoA and relieve the inhibition of carnitine acyl transferase. The two enzymes are regulated in opposite directions in response to insulin, which stimulates fatty acid synthesis and reduces P-oxidation, and glucagon that reduces fatty acid synthesis and increases p-oxidation (Kerner and Hoppel, 2000; Louet et al., 2001; Eaton, 2002).

Fatty acids are the major fuel for red muscle fibers, which are the main type involved in moderate exercise. Children who lack one or the other of the enzymes required for carnitine synthesis, and are therefore reliant on a dietary intake, have poor exercise tolerance, because they have an impaired ability to transport fatty acids into the mitochondria for p - oxidation. Provision of supplements of carnitine to the affected children overcomes the problem. Extrapolation from this rare clinical condition has led to the use of carnitine as a so-called ergogenic aid to improve athletic performance.

Natural Weight Loss

Natural Weight Loss

I already know two things about you. You are an intelligent person who has a weighty problem. I know that you are intelligent because you are seeking help to solve your problem and that is always the second step to solving a problem. The first one is acknowledging that there is, in fact, a problem that needs to be solved.

Get My Free Ebook


Post a comment