A dehydrogenase is an enzyme that removes hydrogen ions from a molecule, thereby oxidizing it.Lactate dehydrogenase converts lactate to pyruvic acid and NADH to NAD+ in glycolysis.The reaction is reversible, in which case it provides pyruvate for gluconeogenesis.Lactate dehydrogenase is present in most tissues with some isoforms more frequent than others. For example, isoenzyme LDH-1 is found in the heart, red blood cells, and brain.

We're told in the question stem that phosphofructokinase is an important regulatory enzyme for the glycolysis pathway. In this pathway, glucose is partially oxidized to provide energy for the cell. Therefore, when the cell has plenty of energy available to meet its metabolic needs, the activity of phosphofructokinase will be reduced in order to suppress glycolysis. Alternatively, when the cell is in need of energy, it will turn glycolysis on by increasing the activity of this enzyme. Thus, compounds that indicate the cell has a lot of energy available, such as ATP and NADH, would be expected to be allosteric inhibitors of this enzyme. So, if the cell has a low amount of ATP, the energy carrying molecule, we would expect the activity of this enzyme to be high. High levels of NADH and citric acid cycle intermediates signal that the cell has enough energy, therefore these would serve to reduce the activity of this enzyme. Low levels of ADP would signal that the cell most likely has a high amount of ATP and is thus in a state of energy surplus. Consequently, this scenario would likely reduce the activity of this enzyme.

When there are high levels of ATP in the blood, ATP itself can act as a signal for the inhibition of ATP production. phosphofructokinase-1 (PFK-1) and pyruvate kinase are major sites of glycolytic regulation. ATP can inhibit these enzymes by binding to their allosteric sites. If these allosteric binding sites are lost, ATP can never bind, and glycolysis will continue indefinitely. Conversely, glycolytic intermediates (like fructose-1,6-bisphosphate) and glycolytic activators (like fructose-2,6-bisphosphate) can act on the same enzymes to increase their activity. Loss of allosteric binding sites for fructose-1,6-bisphosphate and fructose-2,6-bisphosphate on pyruvate kinase and PFK-1, respectively, will result in the slowing down or inhibition of glycolysis.

Glycolysis will be stimulated in situations that require the body to make more ATP.  When the pH is low, ATP is depleted, AMP is at high levels, and carbon dioxide is increased, the body is likely going to need more of an energy supply.  All of these are related to exercise - a situation in which more ATP will be required.  However, an increase in levels of pyruvate implies that glycolysis is actually backed up and should not be stimulated.

While phosphofructokinase-2 is responsible for creating fructose-2,6-bisphosphate, this molecule will actually upregulate the enzymatic activity of phosphofructokinase-1 (PFK-1). As a result, fructose-2,6-bisphosphate is responsible for increasing the amount of glycolysis done in cells via activation of the glycolytic enzyme PFK-1. 

Feedback inhibition is a process by which the products of a reaction or series of reactions slows, stops or inhibits one of the previous reactions in the process, thereby controlling the rate of reaction, and rate of formation of the products.

In glycolysis, one of the end products is energy in the form of ATP. ATP acts as an inhibitor of phosphofructokinase-1, one of the main rate limiting enzymes in glycolysis.