Transport metabolism

THE LIVER PLAYS A CENTRAL ROLE IN LIPID TRANSPORT METABOLISM... 

For the last forty years, many reports have emerged on the hormone-like effects of chemical compounds such as pesticides and industrial chemicals upon wildlife and humans. The effects of these materials are believed to be either direct or indirect. Direct effects involve positive or negative interactions with the hormone receptors. Indirect effects may result when the synthesis of hormones or their receptors is altered, or the transport, metabolism, or elimination of hormones is modified in some way. The discovery of hormone-like properties of some compounds was made long after their release into the environment. It was shown soon after their introduction that aviation crop dusters handling DDT had low sperm counts and workers at a plant producing the insecticide kepone were reported to have low libido, sperm counts and to be impotent. Subsequently, experiments conducted in laboratory animals demonstrated unambiguously the oestrogenic activity of these pesticides. 9 refs. 

By affecting the synthesis, transport, metabolism, and excretion of hormones, thus altering the concentration of normal hormones... 

Occurrence of complex dissimilar actions is thought to be rare at low exposure (ADI) levels but it should always be considered whether a plausible hypothesis exists for effect interactions of two or more compounds. Interactions can occur both in the toxicodynamic phase (e.g., endocrine disrup-tors) and in the toxicokinetic phase (e.g., interference with transport, metabolism (activation, deactivation), distribution, and elimination of another compound). 

Degradative enzymes Resistance of mucosal barrier Membrane transporters Metabolism at mucosa pH changes Disease conditions... 

At the same time, non-linear mechanisms may also possibly be involved in the transport, metabolism, repair and elimination processes involved in mutagenesis (Hoel et oL, 1983). A single threshold step in such a sequence would suffice to give the overall process a threshold. Furthermore, even if mutagenesis at low doses involved a combination... 

FIGURE 66.1 Vitamins D3 and D2 are produced by ultraviolet irradiation of animal skin and plants, respectively. The precursor of vitamin D3 in skin is 7-dehydrocholesterol, or provitamin D. In humans, the storage, transport, metabolism, and potency of vitamins D2 and D3 are identical, and the net biologic activity of vitamin D in vivo results from the combined effects of the hydroxylated derivatives of vitamins D2 and D3. 

Amines labelled with positron-emitting nuclides (11C, 13N, 18F) combined with PET are useful for external measurement of the rate of transport, metabolism and excretion of a number of substances in humans and animals, and were used for various aims. After a detailed study of the organ distribution and the metabolic fate of the 13N-amines, 13N-/ -phenethylamine, 13N- -octylamine and 13N-3,4-dimethoxyphenethylamine, a general transformation path of amines in vivo has been formulated292 (equation 130). 

AhsorptioUf Transport Metabolism, and Excretion Dietary manganese is absorbed from the small intestine by mechanisms that may have a pathway common to that of... 

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