TOKYO, March 12 (Reuters) - Japan's Nikkei share average fell to a one-week low on Wednesday as investors locked in profits after a recent rally and as caution over Chinese demand continued to hurt such commodity-related shares such as mining and metal companies. The Nikkei ended 2.6 percent lower at 14,830.39 points, the lowest closing level since March 4, and the biggest daily drop in more than a month. It had hit a five-week high of 15,312.60 last Friday. The broader Topix index shed 2.1 percent to 1,206.94, with all of its 33 subindexes in negative territory. The JPX-Nikkei Index 400, a gauge comprising firms with high return on equity and strong corporate governance, dropped 2.2 percent to 10,917.03.