Intracellular compartmentation and transport of metabolites

Abstract

The intracellular locations of enzymes and metabolites were determined for ornithine metabolism in Neurospora. Pulse label experiments were used to measure the rates of intracellular translocations and the sizes of compartmented pools of metabolites in the mitochondrial, cytosolic and vesicular compartments. The results indicate that rapid equilibration occurs between these pools during growth in minimal medium, although the vast majority of the ornithine is confined to the vesicular compartment. Arginine, the biosynthetic end‐product of ornithine metabolism, regulates ornithine utilization through a combination of feedback inhibition, repression, and control of intracellular translocations. The last phenomenon plays a decisive role indicating that the regulation of intercompartmental translocations may be a common mechanism in rapid adaptation responses in eukaryotic cells.

H+-ATPases from mitochondria, plasma membranes, and vacuoles of fungal cells

“…The total electrochemical potential thus appears to be 100-110 mV. Under some growth conditions small molecules such as arginine and ornthine accumulate in vacuoles at concentrations 1000-fold higher than in the cytosol [47,163,164]. Their uptake is presumably driven by the difference of electrochemical potential [50].…”

“…Two functions are known for the vacuoles. First, they store high concentrations of polyphosphate, arginine, ornithine, and lysine, and have an important role in the regulation of amino acid metabolism [43,45,47,78,156,[163][164][165]169]. Second, the vacuoles presumably function as lysosomes, because they contain most of the proteases and other hydrolytic enzymes found in the cell [95,156].…”

Compartmentation of uracil in Euglena gracilis.

“…However, this lag period was much smaller than expected on the basis of extractable pyrimidine content, a discrepancy indicative of precursor compartmentation (2). Events participating in this metabolism and compartmentation of uracil in E. gracilis are summarized in a tentative scheme (Fig.…”

“…Such separation between a storage and a metabolically active pool often occurs in different eucaryotic cells. It was originally observed for amino acids in yeast cells (7,31) and was described for Neurospora crassa (2,6,22), alkaloid-producing Penicillium cyclopium (16), plants (17), and, in a modified form, animal cells (5,9). However, the isolation of the storage pool is restricted so far to few examples (14,22,31).…”

“…A delayed incorporation of labeled precursors into products that is much less than expected on the basis of the extractable content of the substance in question is generally accepted as indicator of precursor compartmentation (2). Thus, in E. gracilis, compartmentation of uracil can be measured with two separate products by following its degradation as well as incorporation into RNA.…”