Plasma proline kinetics and the regulation of proline synthesis in man.
A quantitative exploration of the regulation of plasma proline concentration, proline oxidation, and proline endogenous biosynthesis was undertaken utilizing a 360-minute primed continuous infusion of L-[1-13C]proline and L-[methyl-2H3]leucine in healthy, postabsorptive young men. The response of proline metabolism to the intravenous administration of two physiologic rates of L-proline, as well as the withdrawal of an L-proline infusion, were examined. The administration of L-proline at 20 mumol.kg-1.h-1 after an overnight fast resulted in a higher steady state plasma proline concentration, attained within 100 minutes, and this was associated with an increase in proline oxidation, from a baseline value of 10.9 to 16.1 mumol.kg-1.h-1 (P less than .01). Additionally, there was a decrease in proline endogenous synthesis from 15.8 (baseline) to 5.3 mumol.kg-1.h-1 (P less than .01). Administration of L-proline at 40 mumol.kg-1.h-1 after an overnight fast resulted again in a higher plasma steady state proline concentration, attained within 100 minutes and with an associated increase in proline oxidation from 13.1 to 20.0 mumol.kg-1.h-1 (P less than .01) and with a decrease in proline endogenous synthesis from 12.2 to -0.6 mumol.kg-1.h-1 (P less than 0.01). The withdrawal of L-proline after a 20 mumol.kg-1.h-1 infusion resulted in a lower plasma steady state proline level and this was accompanied by a decrease in proline oxidation from 21.2 to 18.2 mumol.kg-1.h-1 (P less than .05) and an increase in endogenous synthesis from 22.2 to 29.7 mumol.kg-1.h-1 (P less than .01).(ABSTRACT TRUNCATED AT 250 WORDS)