Metabolism of glutamine and glutamate in monkey lenses

In rat lenses, glutamine (GLN), not glutamate (GLU), from the surrounding fluids is the primary source of GLU utilized by several metabolic pathways. To study lenticular amino acid metabolism in a primate, fresh lenses from young (2-3 yr) rhesus monkeys (Macaca mulatta) were incubated at 370C for 3, 6, or 24 hr in balanced salt medium containing 5 mM of amino-labeled 15N-GLN or 15N-GLU. The % enrichment of 15N in several free amino acids was determined by GCMS. GLN entered the monkey lenses more rapidly than GLU, but, in contrast to rat lenses, 15N-GLN did not more rapidly label other amino acids. The % of 15N in the (GLN + GLU) pool of the monkey lenses in 15N-GLN reached 20, 35, and 60% at 3, 6, and 24 hr respectively, compared with 10, 20, and 40% in the lenses in 15N-GLU. However, in monkey lenses incubated 24 hr with 15N-GLN, the 15N in alanine, serine, proline, and (aspartate + asparagine) was only 35, 6, 7, and 30% respectively, compared with 50, 10, 7, and 50% in monkey lenses with 15N-GLU. Compared with rat lenses, monkey lenses showed slower transport, deamidation, and metabolism of GLN, and less serine, proline, and glycine synthesis. Also, part of the GLU in monkey lenses appeared to be in a slowly transaminating pool. Species differences should be considered when rats are used as a model to study changes in human lenses during aging and cataractogenesis