Subcellular storage

The results of the tracer studies including the elucidation of the stereochemistry involved, provided a firm basis for a biochemical approach to PA biosynthesis, i. e., characterization of the enzymes that catalyze biosynthetic key steps and the specific mechanisms involved in translocation,subcellular accumulation, and metabolism of PAs. Early tracer work was carried out with intact plants to which tracers were applied for days or weeks. Meanwhile, in vitro plant systems, such as cell cultures and root-organ cultures of PA-producing plants are available. Root cultures were found to be excellent systems for biochemical and enzymatic studies of PA biosynthesis [20-22]. Dedifferentiated cell cultures do not synthesize PAs, but retain the ability to accumulate PAs. They are excellent systems to study the membrane transport of PAs and to identify the subcellular storage sites. 

Once in the serum, aluminium can be transported bound to transferrin, and also to albumin and low-molecular ligands such as citrate. However, the transferrrin-aluminium complex will be able to enter cells via the transferrin-transferrin-receptor pathway (see Chapter 8). Within the acidic environment of the endosome, we assume that aluminium would be released from transferrin, but how it exits from this compartment remains unknown. Once in the cytosol of the cell, aluminium is unlikely to be readily incorporated into the iron storage protein ferritin, since this requires redox cycling between Fe2+ and Fe3+ (see Chapter 19). Studies of the subcellular distribution of aluminium in various cell lines and animal models have shown that the majority accumulates in the mitochondria, where it can interfere with calcium homeostasis. Once in the circulation, there seems little doubt that aluminium can cross the blood-brain barrier. 

Fatty acid utilized by muscle may arise from storage triglycerides from either adipose tissue depot or from lipid stores within the muscle itself. Lipolysis of adipose triglyceride in response to hormonal stimulation liberates free fatty acids (see Section 9.6.2) which are transported through the bloodstream to the muscle bound to albumin. Because the enzymes of fatty acid oxidation are located within subcellular organelles (peroxisomes and mitochondria), there is also need for transport of the fatty acid within the muscle cell this is achieved by fatty acid binding proteins (FABPs). Finally, the fatty acid molecules must be translocated across the mitochondrial membranes into the matrix where their catabolism occurs. To achieve this transfer, the fatty acids must first be activated by formation of a coenzyme A derivative, fatty acyl CoA, in a reaction catalysed by acyl CoA synthetase. 

Metallothionein is a metal storage protein, not a circulating protein. Superoxide dismutases are present in various subcellular particles, for example mitochondria. Peroxisomes contain peroxidases such as catalase. Superoxide dismutases convert the superoxide anion to H202 and are absent from anaerobic microorganisms. 

Storage Changes and Subcellular Freezing Injuries in Recalcitrant Araucaria angustifolia Embryos... 

An important aspect of the preparation and isolation of subcellular particles from brain regions is the criteria by which purity is assessed. Electron microscopy of the various subcellular fractions can provide among the best pieces of evidence for the presence in the preparation of the organelles or subcellular fragments of interest. However, a number of biochemical markers (usually enzymes) that have been established to be present in certain fractions can also be assayed to demonstrate the enrichment of the organelle of interest. For instance, acetylcholinesterase is a common marker for synap-tosomes dopamine-P-hydroxylase is a marker for catecholamine storage vesicles within the synaptosome and cytochrome c oxidase is a marker for mitochondria. Most of the enzymatic markers can be assayed routinely. 

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