Stored Phosphate Metabolism

Similarities between the mechanisms by which the products of proteolysis of the endosperms of castor bean and of maize are processed and transported are evident. The involvement of homoserine may be peculiar to mobilization from the cotyledons of Pisum (or perhaps legumes) since little or no homoserine has been reported in other plants, and the full significance of this interesting digression from the norm is not yet understood. 

There are numerous reports in the literature documenting the existence within monocot and dicot seeds of proteins which specifically inhibit the action of proteinases, particularly those of animal origin [125]. The function of plant proteinase inhibitors is a subject of considerable interest and debate, but it could include one or more of the following  

Storage. Trypsin inhibitors constitute as much as 5-10% of the water-soluble proteins in the embryos and endosperms of barley, wheat, and rye [100]. 

Protection or dissuasion. Proteinase inhibitors might function to inhibit the intestinal proteolytic digestive enzymes of invading insects or the extracellular proteinases of invading microorganisms. 

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Fig. 15 Intracellular phosphatidyl inositol metabolism and postulated roles for inositol phosphates as secondary messengers (adapted from Housley, 1987). (1) One or more of the inositol phosphates (including I(1,4,5)P3) stimulates rapid release of calcium from intracellular stores. (2) Feedback from initial rapid calcium release stimulates intracellular inositol phosphate metabolism. (3) Secondary increase in intracellular calcium levels through activation of membrane calcium-gates and influx of extracellular calcium stimulated by inositol phosphates. PC = phospholipase C.

Triglycerides (or, more appropriately, triacylglycerols) are highly concentrated stores of metabolic energy. They are formed from glycerol 3-phosphate and acylated CoA (Fig. 30.4) and accumulate primarily in the cytosol of adipose cells. When required for energy production, triglycerides are hydrolyzed by lipase enzymes to liberate free fatty acids that are then subjected to p-oxidation, the citric acid cycle, and oxidative phosphorylation. 

In strains dependent on Pit for inorganic phosphate uptake, exposure to arsenate leads to the depletion of intracellular adenosine triphosphate (ATP) stores and the intracellular accumulation of arsenate, demonstrating the direct interference of arsenate in phosphate metabolism (4). 

In humans, thiamine is both actively and passively absorbed to a limited level in the intestines, is transported as the free vitamin, is then taken up in actively metabolizing tissues, and is converted to the phosphate esters via ubiquitous thiamine kinases. During thiamine deficiency all tissues stores are readily mobilhed. Because depletion of thiamine levels in erythrocytes parallels that of other tissues, erythrocyte thiamine levels ate used to quantitate severity of the deficiency. As deficiency progresses, thiamine becomes indetectable in the urine, the primary excretory route for this vitamin and its metaboHtes. Six major metaboHtes, of more than 20 total, have been characterized from human urine, including thiamine fragments (7,8), and the corresponding carboxyHc acids (1,37,38). 

NADP can be converted to nicotinic acid adenine dinucleotide phosphate (NAADP), which has distinct functions in the regulation of intracellular calcium stores. The studies of these new roles of NAD(P) in metabolism are in their early stages, but they might soon help to better understand and explain the symptoms of niacin deficiency ( pellagra) [1]. 

During the recovery period from exercise, ATP (newly produced by way of oxidative phosphorylation) is needed to replace the creatine phosphate reserves — a process that may be completed within a few minutes. Next, the lactic acid produced during glycolysis must be metabolized. In the muscle, lactic acid is converted into pyruvic acid, some of which is then used as a substrate in the oxidative phosphorylation pathway to produce ATP. The remainder of the pyruvic acid is converted into glucose in the liver that is then stored in the form of glycogen in the liver and skeletal muscles. These later metabolic processes require several hours for completion. 

Metabolic acidosis is characterized by decreased pH and serum HC03 concentrations, which can result from adding organic acid to extracellular fluid (e.g., lactic acid, ketoacids), loss of HC03 stores (e.g., diarrhea), or accumulation of endogenous acids due to impaired renal function (e.g., phosphates, sulfates). 

The idea that stimulated inositide metabolism was involved in increases in cytoplasmic Ca2+ was first proposed by Bob Michell (1975). To date, over 20 different inositol phosphates can be isolated from stimulated cells, but so far only one of these molecules can be ascribed a definite function Ins 1,4,5-P3, which is released into the cytoplasm following PLC hydrolysis of PIP2, liberates Ca2+ from intracellular stores. A role for Ins 1,3,4,5-P4in opening a Ca2+ gate , thus allowing the influx of Ca2+ from the external... 

Figure 5-5. Metabolic activities of major organs during a short-term fast. The importance of the liver in providing glucose to support the brain and other glucose-requiring organs in the post-absorptive state is illustrated. The body relies on available glycogen stores as a ready source for glucose as fuel. PPP, pentose phosphate pathway FA, fatty adds TAG, triacylglycerol.

The inositol phosphates are linked into a metabolic cycle (Fig. 6.5) in which they can be degraded and regenerated. Via these pathways, the cell has the ability to replenish stores of inositol phosphate derivatives, according to demand. Ptdins may be regenerated from diacylglycerol via the intermediate levels of phosphatidic acid and CDP-glycerol. 

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