Rod Outer Segment Structure Influences the Apparent Kinetic Parameters of Cyclic GMP Phosphodiesterase

Cyclic GMP hydrolysis by the phosphodiesterase (PDE) of retinal rod outer segments (ROS) is a key amplification step in phototransduction. Definitive estimates of the turnover number, kcat, and of the Km are crucial to quantifying the amplification contributed by the PDE. Published estimates for these kinetic parameters vary widely; moreover, light-dependent changes in the Km of PDE have been reported. The experiments and analyses reported here account for most observed variations in apparent Km, and they lead to definitive estimates of the intrinsic kinetic parameters in amphibian rods. We first obtained a new and highly accurate estimate of the ratio of holo-PDE to rhodopsin in the amphibian ROS, 1:270. We then estimated the apparent kinetic parameters of light-activated PDE of suspensions of disrupted frog ROS whose structural integrity was systematically varied. In the most severely disrupted ROS preparation, we found Km = 95 microM and kcat = 4,400 cGMP.s-1. In suspensions of disc-stack fragments of greater integrity, the apparent Km increased to approximately 600 microM, though kcat remained unchanged. In contrast, the Km for cAMP was not shifted in the disc stack preparations. A theoretical analysis shows that the elevated apparent Km of suspensions of disc stacks can be explained as a consequence of diffusion with hydrolysis in the disc stack, which causes active PDEs nearer the center of the stack to be exposed to a lower concentration of cyclic GMP than PDEs at the disc stack rim. The analysis predicts our observation that the apparent Km for cGMP is elevated with no accompanying decrease in kcat. The analysis also predicts the lack of a Km shift for cAMP and the previously reported light dependence of the apparent Km for cGMP. We conclude that the intrinsic kinetic parameters of the PDE do not vary with light or structural integrity, and are those of the most severely disrupted disc stacks

Updating the Navy's recruit quality matrix an analysis of educational credentials and the success of first-term sailors

This thesis analyzes the impact of different educational credentials on first-term attrition by enlisted sailors in the U.S. Navy. For enlistment screening, the Navy currently categorizes applicants in one of three tiers according to educational attainment. These tiers form the basis of the Recruit Quality Matrix, which employs Armed Forces Qualification Test scores and educational credentials to determine enlistment eligibility. The analysis draws primarily from two sources: a Defense Manpower Data Center file containing enlisted cohorts from fiscal years 1989 through 1997 (to assess first-term attrition), and a Commander, Navy Recruiting Command data base containing enlisted cohorts from fiscal years 1998 through 2003 (to examine bootcamp attrition). Logit regression models are constructed using these data to identify differences in attrition propensities within the general tiers. A refined matrix is designed and evaluated as a more accurate predictor of attrition. Further research is recommended to look at additional measures of success in service, such as performance, productivity, and promotion.http://archive.org/details/updatingnavysrec109451700Lieutenant Commander, United States NavyApproved for public release; distribution is unlimited.Approved for public release; distribution is unlimited