This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Biochemistry
In mammalian tissue, glycine is synthesized from the following precursors: 
Select and explain. 
A. Choline, B. Serine, C. Alanine, D. All of the above
D: All of the above
Answer: D: All of the above 
Explanation: Glycine aminotransferases can catalyze the synthesis of glycine from glyoxylate and glutamate or alanine. Unlike most aminotransferase reactions, these strongly or glycine synthesis. Additional impoant mammalian routes for glycine formation are from choline and from serine. Ref: Rodwell V.W. (2011). Chapter 27. Biosynthesis of the Nutritionally Nonessential Amino Acids. In D.A. Bender, K.M. Botham, P.A. Weil, P.J. Kennelly, R.K. Murray, V.W. Rodwell (Eds), Harper's Illustrated Biochemistry, 29e.