Citrate (citric acid)

The first intermediate of the citric acid cycle. The value is measured in connection with energy metabolism. The value also depends on the acid-base balance, because the kidneys recover citrate to differing degrees depending on the acid load.

What citrate does in the body

In the mitochondria, citrate synthase combines acetyl-CoA – from carbohydrates, fatty acids and some amino acids – with oxaloacetate to form citrate; this starts every round of the citric acid cycle. In the further course, the carbon atoms are broken down step by step to CO₂, and the electrons released go to the respiratory chain as NADH and FADH₂. Citrate has further tasks: exported into the cytoplasm, it supplies the acetyl-CoA for building fatty acids and cholesterol and acts as a signal that slows glycolysis. In the kidneys, citrate is largely recovered; the amount in urine therefore also depends on the acid-base balance. In urine, citrate binds calcium and keeps it in solution.

Low citrate – what happens in the body?

Little citrate can mean that less acetyl-CoA or oxaloacetate is available to start the citric acid cycle – for example when pyruvate dehydrogenase is slowed by vitamin B1 deficiency or oxaloacetate is drawn off for making new glucose. The cycle then turns more slowly, and the respiratory chain receives fewer electrons. In urine, citrate also falls when the body has to compensate for acid – for example with a very high-protein diet or with diarrhoea – because the kidneys then recover more citrate. With less citrate in urine, less calcium stays dissolved, and calcium stones can form more easily in the kidneys.

High citrate – what happens in the body?

A raised value can show that citrate is not being processed promptly in the cycle. The next step, conversion to isocitrate by aconitase, needs an iron-sulphur centre that is easily damaged by iron deficiency and oxidative stress; citrate then builds up in front of this bottleneck, and the following steps receive less supply. Citrate also accumulates when more acetyl-CoA is produced than the cycle can process. In urine, the value rises with an alkaline-rich diet containing plenty of fruit and vegetables, because the kidneys then recover less citrate.

The interpretation is given in your report.