DUBAI- Doha Bank DOBK.QA , Qatar's fifth-biggest lender, has extended by one year a two-year, $575 million bank loan that it had raised in December 2015, but has reduced the size of the facility to $400 million, a bank official said on Tuesday."It was Doha Bank's conscious decision to have the extension at a reduced level of $400 million of the loan amount in order to streamline and optimise our funding mix and to keep the cost (at a) minimum," Kaisar Yusuf Khot, manager of the chief executive's office, said in an emailed response to Reuters questions.Qatari banks have been working to preserve and broaden their sources of overseas funding since Saudi Arabia, the United Arab Emirates, Bahrain and Egypt cut diplomatic and transport links with Qatar last June. The split caused banks from those four states to pull their deposits from Qatari institutions and reduce lending to them.The original loan to Doha Bank, an unsecured facility, was provided by a group of banks including Bank of China, China Construction Bank, Commerzbank, ING Bank, Mizuho Bank and Wells Fargo, which were appointed bookrunners and mandated lead arrangers.Australia and New Zealand Banking Group and Wing Lung Bank participated as mandated lead arrangers, and Commercial Bank of Qatar joined as lead arranger.The loan had a one-year extension option at the lenders' discretion, meaning that at the end of its second year, lenders could decide whether or not to prolong the facility.Doha Bank did not comment on which banks participated in the loan extension. Two sources familiar with the matter said the Chinese, Hong Kong and Japanese banks in the original group chose not to participate, while ANZ, Commercial Bank of Qatar, Commerzbank, ING, and Wells Fargo did take part.(Editing by Andrew Torchia) ((Davide.Barbuscia@thomsonreuters.com; +971522604297; Reuters Messaging: davide.barbuscia.reuters.com@reuters.net))