Carbohydrate challenge tests: do you need to measure methane?

OBJECTIVE: Breath tests that measure hydrogen (H2) have been judged reliable for the detection of lactose maldigestion (LM) and fructose malabsorption (FM). Recently, methane (CH4) testing has been advocated and measurement of CH4 in addition to H2 has been shown to increase the diagnostic accuracy for LM. PURPOSE: This study was designed to consider the additional yield from CH4 measurement in patients tested for LM and FM. METHODS: Patients reported for testing after an overnight fast, not smoking and with their prior evening meal carbohydrate restricted. After challenge with 50 g lactose or 25 g fructose in water, end-alveolar breath samples collected over a 4-hour duration were analyzed for H2 and CH4. Diagnostic positivity was compared using a cutoff level of 20 ppm increase above fasting baseline for H2 alone, which is consistent with consensus guidelines, versus H2 plus twice CH4, which recognizes that CH4 consumes twice the hydrogen. RESULTS: There were 406 LM performed in 93 men and 313 women. Of those tested, 124 (30%) had a positive test for H2 and 139 (34%) had a positive test for H2 + CH4 ×2. There were 178 FM tests performed in 31 men and 147 women. Of those tested, 17 (9%) had a positive test for H2 and 42 (23%) had a positive test for H2 + CH4 ×2. CONCLUSION: If H2 alone was measured without additional CH4 analysis, 4% of patients with LM and 14% patients with FM would not have been identified.

Rapid identification of functionally critical amino acids in a G protein-coupled receptor.

G protein-coupled receptors (GPCRs) comprise one of the largest protein families found in nature. Here we describe a new experimental strategy that allows rapid identification of functionally critical amino acids in the rat M(3) muscarinic acetylcholine receptor (M3R), a prototypic class I GPCR. This approach involves low-frequency random mutagenesis of the entire M3R coding sequence, followed by the application of a new yeast genetic screen that allows the recovery of inactivating M3R single point mutations. The vast majority of recovered mutant M3Rs also showed substantial functional impairments in transfected mammalian (COS-7) cells. A subset of mutant receptors, however, behaved differently in yeast and mammalian cells, probably because of the specific features of the yeast expression system used. The screening strategy described here should be applicable to all GPCRs that can be expressed functionally in yeast.