Enzymes storage compartments

In yeasts and other fungi, the vacuole is an important organelle sharing some properties with the mammalian lysosome (an acidic compartment containing a variety of hydrolytic enzymes) and with the plant cell vacuole (responsible for metabolite storage and for cytosolic ion and pH homeostasis) [18,19]. 

We will consider the structural and mechanistic features of these enzymes by examining the Ca2+ ATPase found in the sarcoplasmic reticulum (SR Ca2+ ATPase) of muscle cells. This enzyme, which constitutes 80% of the sarcoplasmic reticulum membrane protein, plays an important role in muscle contraction, which is triggered by an abrupt rise in the cytosolic calcium level. Muscle relaxation depends on the rapid removal of Ca + from the cytosol into the sarcoplasmic reticulum, a specialized compartment for calcium storage, by the SR Ca + ATPase. This pump maintains a Ca2+ concentration of approximately 0.1 iM in the cytosol compared with 1.5 mM in the sarcoplasmic reticulum. 

The fungal cell contains a vast number of ribosomes, which are usually present in the form of polysomes — lines of ribosomes strung together by a strand of mRNA. Ribosomes are the site of protein biosynthesis. The system that mediates the export of proteins from the cell involves a number of membranous compartments including the Golgi apparatus, the endoplasmic reticulum and the plasmalemma. In addition, the vacuole is employed as a storage space where nutrients, hydrolytic enzymes... 

The subcellular localization of the enzyme is interesting. In contrast to tyrosine hydroxylase, which is localized to the cytoplasmic compartment, dopamine hydroxylase is largely contained within the aminergic storage vesicles. Thus, it appears that dopamine, which is synthesized in the cytosol, must be taken up into these vesicles in order to be converted to norepinephrine. Approximately 50% of the enzyme is associated with the membranous portion of the vesicles, whereas the other 50% is in a soluble form within the vesicle. During synaptic transmission noradrenergic neurons release both norepinephrine and its biosynthetic enzyme by an exocytotic mechanism. While most of the released norepinephrine is taken back up by the terminal, the enzyme is believed to diffuse of the synaptic cleft into the extracellular fluid and eventually into the serum. There are relatively large amounts of dopamine p-hydroxylase in human serum that are believed to arise from release from sympathetic neurons. 

Whole-body analyses offish and other animals do not reflect high concentrations of metals in the liver and pancreas, which can disrupt normal biochemical processes in these tissues. Also, once saturation of a storage system occurs, spillage into other compartments can disrupt enzyme systems and produce toxic effects, without producing any significant increase in whole-body analyses (Roesijadi 1992, Langston et al. 1998, Olsson et al. 1998). 

---