Problem an experiment with C and Nlabelled urea

The first enzyme to be crystallized (and hence the first evidence that enzymes are proteins) was urease, which catalyses the hydrolysis of urea to ammonium and carbon dioxide. The original preparation of urease was from plant material, but the enzyme is also known to occur in a number of bacteria. In this study, a group of volunteers were given an intravenous infusion of 20 mmol urea labelled with both 13C and 15N and their urinary excretion of label was measured over 24 hours. Complete recovery of the label in urine would amount to 40 mmol 15N and 20 mmol 13C. The results are shown in column 2 of Table 9.19. The experiment was repeated a week later, after they had received the antibiotic neomycin for 4 days to sterilize the gut; these results are shown in column 3.

What conclusions can you

draw from these results?

Table 9.19 Recovery of label in urine after intravenous infusion of 20 mmol labelled with 13C and 15N

Recovery (mmol/24 h)

Initial study

After neomycin

Total urine N

609

613

Total urea

500

497

Total urine l5N

34

39

Total urine l3C

0.5

19.5

Urea l5N

29

39

Urea l3C

< 0.1

19.5

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