Active transport bacterial systems

In eukaryotes there is also evidence that Met(O) is actively transported. It has been reported that Met(O) is transported into purified rabbit intestinal and renal brush border membrane vesicles by a Met-dependent mechanism and accumulates inside the vesicles against a concentration gradient102. In both types of vesicles the rate of transport is increased with increasing concentrations of Na+ in the incubation medium. The effect of the Na+ is to increase the affinity of Met(O) for the carrier. Similar to that found in the bacterial system, the presence of Met and other amino acids in the incubation medium decreased the transport of Met(O). These results suggest that Met(O) is not transported by a unique carrier. 

In a cell-free system inhibition can be shown to occur on both 70S and 80S ribosomes. However, in a more realistic in vitro setting intact prokaryotic (i.e., bacterial) cells are much more sensitive. The reason for this selectivity is that tetracyclines are actively transported into bacterial but not mammalian cells. In Gm- bacteria, at least, the more water-soluble compounds seem to cross through membrane channels (pores). The more lipid-soluble drugs (particularly MNC, Table 6-9) diffuse more readily through the lipoidal phases of the membranes. This energy-coupled process then leads to intracellular antibiotic accumulations. 

To prevent the detrimental effects of heavy metals, many bacterial species have evolved a sophisticated and highly regulated detoxification system in which mercurials and Hg (II) are actively transported into the intracellular space, where ultimate reduction of Hg (II) to the much less toxic Hg (o) leads to its elimination from the cell (Stefan and Miller, 1999). 

These differ little in antibadetial activity and are distinguished by their pharmacokinetic behaviour. Minocycline (Rj=N(CH3)2, R2=H, R3=0H, R4=H) shows a broader spedmm of antibacterial activity. The tetracydines ad by blocking the binding of aminoacyl tRNA to the A site on the ribosome. The tetracydines subsequently inhibit protein synthesis at the small rihosomal suhunits of both the 70S (prokaryote) and SOS (eukaryote) tihosome. However, an active transport system for the tetracydines in bacteria means effective concentrations are achieved in the bacterial cdls but not in mammalian cdls. The tetracyclines are presented as capsules and tablets for oral use. 

The processes of electron transport and oxidative phosphorylation are membrane-associated. Bacteria are the simplest life form, and bacterial cells typically consist of a single cellular compartment surrounded by a plasma membrane and a more rigid cell wall. In such a system, the conversion of energy from NADH and [FADHg] to the energy of ATP via electron transport and oxidative phosphorylation is carried out at (and across) the plasma membrane. In eukaryotic cells, electron transport and oxidative phosphorylation are localized in mitochondria, which are also the sites of TCA cycle activity and (as we shall see in Chapter 24) fatty acid oxidation. Mammalian cells contain from 800 to 2500 mitochondria other types of cells may have as few as one or two or as many as half a million mitochondria. Human erythrocytes, whose purpose is simply to transport oxygen to tissues, contain no mitochondria at all. The typical mitochondrion is about 0.5 0.3 microns in diameter and from 0.5 micron to several microns long its overall shape is sensitive to metabolic conditions in the cell. 

Recently, a potential cytosolic component of the MEP precursor pathway, xylulose kinase, has been cloned and tested for function in an Escherichia coli complementation system. " The kinase activates exogenous xylulose in the cytoplasm. DXP is the precursor for DXS, which resides in the plastid, suggesting the activated substrate must be transported into the plastid. Another xylulose kinase homologue in Arabidopsis that contains a plastid targeting sequence was not active in the E. coli system, suggesting that it may have some other function in the plastid. Perhaps plant and bacterial tissue cultures may be fed xylulose to condition accumulation of isoprenoid metabolites. 

---