Specific Research Areas
My laboratory focuses on the role of amino acids (notably glutamine and glutamate) as substrates for gluconeogenesis and urea synthesis. This encompasses work in vitro and in vivo using cells and tissues from many different organs, as well as the use, to date, of thirteen different species. (I am a great believer in the August Krogh principle, "for many problems in physiology there is an animal on which it can most conveniently be studied.")
Tissue-specific glutamine metabolism
I have studied glutamine metabolism in liver, kidney, intestine, skeletal and cardiac muscle, adipose and tissue, immune cells, as well as the lactating mammary gland, and placental and fetal tissues during gestation. We were the first to isolate metabolically viable enterocytes from the rat and to identify and quantitate the major end products of glucose, glutamate and glutamine metabolism in such cells. We developed the first microdialysis and arterio-venous difference techniques to quantify metabolic flux in rat adipose tissue in vivo and established that rat adipose tissue is a site of net glutamine production. We recently established that adipocyte glutamine synthetase is involved in the adipocyte remodeling that occurs in obesity and that adipocytes can provide the glutamine that is essential to macrophage survival. We are continuing these studies with the ultimate aim of defining the role of adipocyte glutamine synthesis in development of insulin resistance and Type 2 diabetes mellitus.
Regulation of phosphate activated glutaminase and glutamine synthetase
My group demonstrated that hepatic (liver-type) glutaminase was subject to long-term hormonal and dietary regulation. We were then the first to purify liver-type glutaminase, raise antibodies to the protein and isolate, and sequence, both cDNA and genomic clones. Using these tools we demonstrated that hormonal and dietary regulation occurred at the level of transcription and identified the upstream enhancer sequences involved. (Note: This gene and protein have been renamed, Gls2, and is currently the focus of extensive investigation in tumor biology). For many years we have also studied the opposing enzyme, glutamine synthetase and have identified how this enzyme is regulated by glutamine induced proteolysis via the ubiquitin linked proteasome. We continue to study the role of this enzyme in adipocyte differentiation (qv), and also in the provision of milk glutamine.
Glutamine supplementation in lactation
Glutamine and glutamate are the most abundant amino acids in milk. We established that lactation in is accompanied by a net catabolic state and loss of lean body mass, caused, in part, by the need to provide glutamine in the milk. We recently showed that glutamine and glutamate supplementation to the diet can both prevent the loss of lean body mass during lactation and also increase the glutamine content of milk. We are extending this work to the study of amino acid metabolism during lactation and the role of different tissues in the provision of milk glutamine and glutamate.
Glutamine metabolism in uricotelic species
One of my long standing interests is the comparative biochemistry/physiology of nitrogen excretion. To this end we have carried out extensive studies of uric acid production in species such as birds and lizards.
Outstanding theoretical questions
I am continuously searching for ways to address the following questions (but the answers are far from obvious and certainly not easy to test experimentally):
- Why do rapidly dividing and differentiating cells require the catabolism of extremely large amounts of glutamine?
- Are glutamine and glutamate dispensable or indispensable in the diet?