Mouse mammary carcinoma porphobilinogenase and hydroxymethylbilane synthetase

1. Porphobilinogenase (PBGase) and hydroxymethylbilane synthetase (HMB-S) were investigated in crude extracts from mouse mammary carcinoma, normal mouse liver and tumor bearing mouse liver. 2. A Michaelis-Menten kinetics for both enzymes in either source was observed. Km values of 87 to 108 microM, Vmax of 1.57-1.83 nmol porphyrins/2 hr and a Hill coefficient of n = 1 were obtained for PBGase and Km values of 13 to 19 microM and Vmax of 2.6-4.8 were obtained for HMB-S. 3. Porphyrin synthesis was linear up to 180 min of incubation in all cases for PBGase and HMB-S, and greatly increased with higher incubation temperature being maximal at 60 degrees C and nil at 70 degrees C, optimal temperature was 37 degrees C for either enzyme in either source. 4. Uroporphyrinogen III synthetase was heat inactivated while HMB-S was a heat stable enzyme. Optimum pH was 8.2 for PBGase and HMB-S in either tissue.