Which of the following transports nitrogen from muscle to liver -

Ans. is 'b' i.e., Alanine o Transport of alanine from muscle to liver (glucose-alanine cycle) has two functions :-i) Providing substrate for gluconeogenesisii) Transport of ammonia (NH4-) to liver for urea synthesis.o During the first few days of starvation, there is a rapid breakdown of muscle protein, providing aminoacids for gluconeogenesis. The most important gluconeogenic amino acid is alanine Q (through glucose-alanine cycle). Alanine is released due to increased muscle protein breakdown Q and is transported to liver for gluconeogenesis (glucose alanine cycle). The other important gluconeogenic amino acid in starvation is glutamine.o After some weeks of starvation, the rate of muscle breakdown decreases due to decreased need of glucose as a fuel for brain which has began using ketone bodies as a source of energy. However, when the triacylglycerol stores are completely exhaused, muscle proteins once again enter in a stage of rapid depletion due to protein breakdown.o Alanine transport from muscle to liver has an important additional role, beside gluconeogenesis. Because muscle is incapable of synthesizing urea, most of the ammonia formed by protein catabolism is transferred to pyruvate to form alanine by transamination (PGI-97). Alanine is transported to liver where amino group of alanine is removed to form urea and the resulting pyruvate is converted to glucose by gluconeogenesis.o Thus, sources of alanine release from muscle are (i) Degradation of protein (ii) Transamination of pyruvate.o Besides alanine, lactate produced in muscles by anerobic use of glucose, also seves as a substrate for gluconeogenesis-cori cycle or lactic acid cycle.