Physiology, question regulation

Next, I would like to raise the question whether any control mechanisms operate past the mevalonate stage. I am unaware of any experimental evidence to date for a physiologically significant regulation of the intermediate steps between mevalonic acid and cholesterol, either by repression, induction or allosterically. 

The 11,12-oxido and 14,15-oxido analogs of leukotriene A4 were synthesized to help answer the question of whether these compounds might be biosynthesized from arachidonate by the 12- and 15-lipoxygenation pathways and serve as physiologic regulators. Hydrolysis products of the 14,15-oxide were later found to be formed in biological systems. 

The existence of two distinct functions within the one membrane protein raises a number of interesting questions about transporter functions, such as what is the relative importance of the two functions Can they be allosterically regulated under physiological conditions or manipulated by pharmacological methods What is the structural basis for the dual functions In the next section, we shall attempt to address some of these issues and also to present some of the current understanding of the structural basis for these functional states. 

These data on Ca0 effects observed in the low-resistance epithelia raised a question related to their physiological relevance, in light of the tight regulation of extracellular Ca2+ level at 1.2-1.5 mM range. The extracellular Ca2+ level at the cell surface is likely to vary because of the presence of cell surface or cell secreted Ca2+-binding proteins [106, 107]. Therefore, the experimental EC50... 

These simple facts directly raise the issue of metabolic control or regulation. What factors control those aspects of our metabolism that are actually expressed at some point in time and under some set of conditions This is an enormously important question and we will see an example later. For the moment, we note that regulation is performed by a delicate set of interrelated checks and balances. These have both intrinsic (i.e. genetic) and extrinsic (i.e. physiological, environmental) components. 

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