Date of Award
1991
Degree Type
Dissertation
Degree Name
Doctor of Philosophy
Abstract
Corticosteroid binding globulin (CBG) is the major plasma transport protein for glucocorticoids. Complementary DNAs for rat, mouse, rabbit and squirrel monkey CBG were isolated, sequenced and their deduced primary structures were aligned. Overall, they exhibit 40.9% identity, and their comparison revealed conserved N-glycosylation sites and the probable position of the cysteine located within the human CBG steroid binding domain. Rat CBG mRNA is produced primarily by the liver, and in adults, males have two-thirds the CBG mRNA levels of females. During the last third of pregnancy, maternal hepatic CBG mRNA levels are relatively constant while fetal levels are highest at day 15 of gestation and decline to barely detectable levels at term. Concentrations of CBG mRNA are very low at birth, and adult values are attained by puberty. However, serum CBG concentrations do not reach adult levels until 6 weeks of age, and this is probably due to the relatively short half-life of CBG in infants (6.9 h) when compared to adults (14.5 h). Dexamethasone reduces rat serum CBG and hepatic CBG mRNA levels by 4 and {dollar}>{dollar}26-fold, respectively, and the latter is the result of reduced CBG gene transcription. Thyroxine tends to increase serum CBG and hepatic CBG mRNA levels, but not CBG gene transcription. Sepsis reduces rat hepatic CBG mRNA levels to 11% of normal, and this is reflected by low serum CBG levels. Rat and human carriers of CBG variants with reduced cortisol binding activity have been identified, and sequence analysis of BioBreeding rat CBG cDNAs and exons of the human CBG gene revealed methionine{dollar}\sp{lcub}276{rcub}{dollar} to isoleucine{dollar}\sp{lcub}276{rcub}{dollar} and leucine{dollar}\sp{lcub}93{rcub}{dollar} to histidine{dollar}\sp{lcub}93{rcub}{dollar} substitutions in rat and human CBG, respectively. Scatchard analysis of media obtained from Chinese hamster ovary cells transfected with normal and mutant cDNAs for rat and human CBG confirms amino acid substitutions at these locations are responsible for reduced steroid binding affinity. Codons for other amino acids suspected to contribute to the steroid binding activity of human CBG have been mutated in vitro and the resulting cDNAs were expressed in culture. Analysis of these products should further contribute to our understanding of the relationship between CBG structure and function.
Recommended Citation
Smith, Carolyn L., "The Structure, Biosynthesis And Function Of Corticosteroid Binding Globulin" (1991). Digitized Theses. 2099.
https://ir.lib.uwo.ca/digitizedtheses/2099