Metabolism storage pools

Judged by enzyme induction, Menary and Jones (1972) indicated that nitrate was unable to move from a storage pool (vacuole) to the cytoplasm of ripe paw paw fruit. Using a modified in vivo assay, Ferrari et al. (1973) estimated sizes of active (metabolic) and inactive (storage) pools of nitrate in tobacco cells, barley aleurone layers and maize leaves. The location and nature of the pools was not defined. 

The content of hemicelluloses drops from 9% to 1-2% (relative to fresh weight), hence they act as a storage pool in carbohydrate metabolism. There is also a drop in the sugar content in bananas during the post-climacteric stage. 

It is already evident that the turnover rate of a transmitter is only a crude measure of its release rate. Further limitations are that there is appreciable intraneuronal metabolism of some neurotransmitters notably, the monoamines. In such cases, turnover will overestimate release rate. Another problem, again affecting monoamines, is that some of the released neurotransmitter is taken back into the nerve terminals and recycled. This leads to an underestimate of release rate. Despite these drawbacks, studies of turnover rates uncovered some important features of transmitter release. In particular, they provided the first evidence for distinct functional pools of monoamines, acetylcholine and possibly other neurotransmitters a release pool, which could be rapidly mobilised for release, and a storage or reserve pool which had a slower turnover rate. 

A central feature of protein metabolism is the amino acid pool, consisting of amino acids in the bloodstream. Figure 4.5 illustrates the metabolic relationship of the amino acid pool to protein breakdown, synthesis, and storage. 

Metal bridges are formed from enzymes or hormones to the substrates. These intermediate complexes are able to enhance the reaction in a number of metabolic processes, 3. Some metal ions are known to act as a cofactor or coenzyme being involved in redox reactions directly. Further, metalloproteins may serve as a pool for metal storage. Examples for this are displayed by ferritin, transferrin or ferrichrome, to mention just a few. 

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