What is Lactate dehydrogenase's ( LDH ) function?

[Rembrandt]

Hi all,

One of my laboratory assignments was to determine the enzym activity of lactate dehydrogenase (LDH). 

When doing a little wikipedia research I got a wee bit confused. I interpreted the following:
LDH converts lactate to pyruvate and vice versa.   Lac + NAD⁺   <-->   PYR + NADH

Then comes the Cori cycle:
in very active muscle cells, Pyr is converted to LAC, so the LAC can be transported to the liver where it will still be converted to PYR, and then to glucose.

I do not understand why? Why bother converting pyruvate if it is needed as pyruvate in the liver?

I thought pyruvate was also needed in the citric acid cycle? so why then making lactate out of it in the first place?

Lactate is already made by anearobic glycolysis in the cells (converting glucose to lactate), right? so why make even more?

It makes sense to me that you would want to make pyruvate out of lactate, since this could be a waste product of the anaerobic respiration, but why the other way around.

 

I hope you guys and gals can make me a little wiser. Thanks in advance for the efforts!

Kind regards,
Rembrandt

[Rembrandt]

After some deeper searching it seems to me that the pyruvate to lactate conversion takes place in cells because the  NAD⁺ that is formed is needed for the citric acid cycle and glycolysis.

For these metabolic pathways, NAD⁺ is required to make ATP. But under heavy duty, muscular cells cannot be supplied quickly enough with sufficient amounts of NAD⁺. 
So that is were LDH steps into the game. LDH converts pyruvate to lactate, doing so also converts NADH to NAD⁺, which is a very good thing for the glycolysis and citric acid cycle. 

After LDH has temporarily supplied the cell with NAD⁺, the lactate that was formed during this conversion by LDH, but also the lactate that may have formed by anaerobic fermentation of glucose, will be transported to the liver. There the lactate will be converted back to pyruvate (also by LDH) and later on the glucose again. 

could somebody with more know-how of this stuff maybe confirm this?

And another question: why did we need to buffer the solution with potassiumphosphate (pH=7,5)? 

I suspect that is has to do with the NADH hydride that attacks pyruvate, but i dont know how.

[Rembrandt]

Or maybe stabilization of the positive formal charge on the Nitrogen in NAD?

[BabcockHall]

Are enzymes equally active at all pH values, or do they have an optimum pH?  Does the balanced reaction produce/consume protons?  The mechanism that you drew implies the answer.  In other words if one used unbuffered water, would the pH of the water gradually change with time.

 

With respect to why convert pyruvate into lactate, there are several ways to think about it.  If a muscle cell did not, what would happen to the ratio of NAD to NADH within that cell? 

Edited April 29, 2020 by BabcockHall
added one sentence

[BabcockHall]

On 4/9/2020 at 11:50 PM, Rembrandt said:

I thought pyruvate was also needed in the citric acid cycle? so why then making lactate out of it in the first place?

Lactate is already made by anearobic glycolysis in the cells (converting glucose to lactate), right? so why make even more?

It makes sense to me that you would want to make pyruvate out of lactate, since this could be a waste product of the anaerobic respiration, but why the other way around.

Muscle cells undergoing anaerobic consumption of glycogen produce more pyruvate than they can consume aerobically via the TCA cycle.  The pyruvate is instead converted into pyruvate.  Your second sentence implies that there is an additional way to make lactate, but there isn't.

[BabcockHall]

16 hours ago, BabcockHall said:

 The pyruvate is instead converted into pyruvate.

Typo.  The pyruvate is instead converted into lactate.